state.cppm: restore PrepareState's doc comment, fix a stale one

The comment originally attached to the PrepareState struct definition
stayed behind in prepare.cppm when the struct itself moved to
state.cppm (it was written above the struct's enclosing namespace, not
the struct itself), leaving state.cppm's copy undocumented. Restored,
updated for the current file layout. Also fixed a comment that had
gone stale during the P9/P11 split: fp/stdBmiPath/scan/report and
their siblings are computed inside P9 (target_side.cpp) and P11
(scan.cpp) themselves, not inside P2's resolve_target_toolchain closure
as an earlier commit's comment claimed.

No code change; golden fixture diff against the baseline is empty.

Author: speak-agent

src/build/prepare/state.cppm

@@ -264,6 +264,26 @@ constexpr std::size_t kMainConsumer = static_cast<std::size_t>(-1);
         std::string url, refKind, ref;
     };
 
+// PrepareState carries prepare_build's working state across the phases it
+// decomposes into (see the layout comment at the top of prepare.cppm). Each
+// phase is an ordinary function taking `PrepareState&`; the state itself is
+// constructed once, in driver.cpp's prepare_build(), and lives for the
+// whole call — including across a phase that stores a closure for a LATER
+// phase to call (resolve_target_toolchain, defined in toolchain.cpp's P2 and
+// called from its own P5): such a closure captures `state` itself rather
+// than individual locals, so it stays valid no matter which phase's stack
+// frame created it.
+//
+// A member exists here because some phase after the one that computes it
+// still reads it, or because a stored closure needs it to remain valid past
+// its own phase — several members exist ONLY for that second reason and are
+// never read by name from another phase (bootstrap_checked, kMainConsumer's
+// siblings). A value read and written within a single phase stays an
+// ordinary local in that phase's function body; it does not move here.
+//
+// Not copied: copying this by value would copy every dependency-graph and
+// plan structure prepare_build ever builds, silently, at whichever call
+// happened to pass it by value instead of by reference.
 struct PrepareState {
     PrepareState(bool print_fingerprint_, bool includeDevDeps_,
                  std::vector<mcpp::manifest::Target> extraTargets_,
@@ -486,8 +506,8 @@ struct PrepareState {
     bool targetSideResolved = false;
     mcpp::modgraph::UsageRequirements targetSideUsage;
 
-    // ── computed inside resolve_target_toolchain (P2's closure), read by P9
-    // and P13 well after that closure returns ───────────────────────────────
+    // ── P9/P11: the module scan, its validation, and the fingerprint they
+    // feed; read again by P13 (the plan, resolution.json) ───────────────────
     mcpp::toolchain::Fingerprint fp;
     std::filesystem::path stdBmiPath;
     std::filesystem::path stdObjectPath;