fix(xml): XmlConvert's TimeSpan pair uses the XSD duration form (#2080, SR-AUD-354)

XmlConvert::ToString(TimeSpan) emitted .NET's NATIVE colon form --
"1.00:00:00.0000000" for one day -- and ToTimeSpan parsed it. .NET's are
`new XsdDuration(value).ToString()` and `new XsdDuration(s).ToTimeSpan()`, and
never look at the colon form.

The deferral named three sub-questions with no repository-contained answer. The
reference answers all three.

(1) The year/month factors: XsdDuration ESTIMATES -- 365 days to the year, 30 to
the month -- and says so in as many words. Reproduced rather than improved on: a
"better" estimate would disagree with every .NET-produced value. P12M rounds to
a year, because the algorithm is (years + months/12)*365 + (months%12)*30.

(2) Does the colon form stay accepted: no. The parser requires a leading P.
Accepting both would make the contract "either grammar", which no reference or
schema defines.

(3) The exception: FormatException, always, because XmlConvert.ToTimeSpan wraps
every XsdDuration failure -- including its own OverflowException -- in one.

Two things surprised the first cut and are recorded rather than smoothed over.
"PT.5S" is VALID: .NET's '.' branch is the one component that does not check for
preceding digits, and XML Schema's duSecondFrag admits ('.' fracFrag) -- the
test was wrong, not the parser. And one line of the reference is DEAD:
XsdDuration.TryParse's trailing `if (numDigits != 0)`, which every reaching path
has already made unreachable. The port omits it, and a mutation confirmed that
changes nothing.

ToString and ToTimeSpan round-trip, asserted over eight values including
negatives, zero and a single tick.

Two #1836 tests are rewritten in the new grammar rather than deleted, because
the property they pin -- an out-of-range day count raises rather than wrapping
negative -- is unchanged.

+3 net tests. Five mutations: three caught directly, one re-run after an invalid
first cut, one that led to deleting the dead line.

Gate: 17,265 run, 0 failed, 38 executables -- green. Downstream, measured: zero
XmlConvert sites in either consumer.

SR-AUD-354 -> remediated. Record: docs/Migration-XmlConvertXsdDuration.md.

Author: robertvokac

CLAUDE.md

@@ -0,0 +1,78 @@
+<!-- SPDX-License-Identifier: MIT -->
+<!-- Copyright (c) Robert Vokac and contributors -->
+
+# Migration — `XmlConvert` TimeSpan uses the XSD `duration` form (ticket #2080)
+
+*2026-08-17.* `XmlConvert::ToString(TimeSpan)` emitted .NET's **native colon form** and
+`XmlConvert::ToTimeSpan` parsed it. .NET's use the XML Schema `duration` lexical form and never
+look at the colon form at all.
+
+Landed under `docs/StandingApprovals.md` SA-5. No public signature, layout, vtable or `noexcept`
+change. **This changes emitted text and narrows accepted input** — read §1.
+
+---
+
+## 1. What changed
+
+| Call | Was | Is |
+|---|---|---|
+| `ToString(TimeSpan::FromDays(1))` | `"1.00:00:00.0000000"` | `"P1D"` |
+| `ToString(TimeSpan::FromTicks(0))` | `"00:00:00"` | `"PT0S"` |
+| `ToString(TimeSpan::FromMinutes(90))` | `"01:30:00"` | `"PT1H30M"` |
+| `ToString(TimeSpan::FromDays(-1))` | `"-1.00:00:00"` | `"-P1D"` |
+| `ToTimeSpan("P1D")`, `ToTimeSpan("PT1H30M")` | `FormatException` | parsed |
+| `ToTimeSpan("1.00:00:00")` | parsed | **`FormatException`** |
+| `ToTimeSpan("PT.5S")` | `FormatException` | 0.5 s — see §3 |
+
+`ToString` and `ToTimeSpan` round-trip, and a test asserts that over eight values including
+negatives, zero and a single tick.
+
+## 2. Why, and the three questions the deferral could not answer
+
+The deferral listed exactly three sub-questions with no repository-contained answer. The
+reference answers all three:
+
+* **The year/month conversion factors.** `XsdDuration` *estimates* — 365 days to the year, 30 to
+  the month — and says so in as many words (`XsdDuration.cs:243-244`). Reproduced rather than
+  improved on: a "better" estimate would disagree with every .NET-produced value. Note `P12M`
+  rounds to a year, because the algorithm is `(years + months/12) * 365 + (months % 12) * 30`.
+* **Whether the native colon form stays accepted.** It does **not**. `XsdDuration`'s parser
+  requires a leading `P`, and `XmlConvert.ToTimeSpan` goes straight to it
+  (`XmlConvert.cs:1109-1127`). Accepting both would make the method's contract "either
+  grammar", which no reference or schema defines.
+* **The exception identity.** `FormatException`, always. `XmlConvert.ToTimeSpan` wraps *every*
+  `XsdDuration` failure — including its `OverflowException` — in one, with the comment "Remap
+  exception for v1 compatibility" (`:1118-1122`).
+
+## 3. Two details that surprised the first cut
+
+**`PT.5S` is valid.** .NET's `'.'` branch is the one component that does **not** check whether
+any digits preceded it, and XML Schema agrees: `duSecondFrag` admits `('.' fracFrag)` with no
+leading digit. The first cut of the test listed `PT.5S` as malformed; the expectation was wrong,
+not the parser, and there is now a row recording that so it is not "fixed" later.
+
+**One line of the reference is dead.** `XsdDuration.TryParse` ends with
+`if (numDigits != 0) goto InvalidFormat;` before its "no trailing characters" check. Every path
+that reaches it has already rejected `pos >= length`, so `pos < length` holds and the next check
+rejects regardless. The port omits it, and a mutation confirmed the omission changes nothing.
+
+## 4. Two tests from ticket #1836 are rewritten, not deleted
+
+`#1836` pinned that an out-of-range day count *raises* rather than wrapping to a negative
+duration. That property is unchanged and still asserted — in the new grammar, and with
+`FormatException` rather than `OverflowException`, which is .NET's own remapping.
+
+## 5. To migrate
+
+If you write XML, this is the fix: `xs:duration` is what a schema-validating consumer expects,
+and the colon form was never valid there.
+
+If you were round-tripping through these two methods, you still are — the pair is consistent.
+
+If you were feeding `XmlConvert::ToTimeSpan` a colon-form string from elsewhere, use
+`System::TimeSpan::Parse`, which is the method for that grammar and is unchanged.
+
+## 6. Downstream, measured
+
+Neither `cna` nor `mobile-eggbert` references `XmlConvert` or `System::Xml` — **zero sites in
+both**. Neither repository was modified.