Decide whether a node type may be defined outside the kernel (#1026) (#1040)

Entity is a public abstract record with a protected constructor, which reads
as an invitation to derive from it, and five of its eleven abstract members
are internal or private protected, so a subclass in another assembly fails to
compile with CS0534 naming exactly those five. That is the blocker under #746
items 53 and 56 and under Packaging.md's "blocked, not decided" for domain
packages -- item 56 attributes the seam to #338, looking a type up so it can be
parsed, which is the second half of it.

The decision is to open the contract without publishing anything: four of the
five are engine machinery with a default the kernel can supply and be right
about -- Priority is read only by the printers, SortHashName is a deduplication
table over kernel node kinds, ToSymPy already throws from a base class for a
node SymPy has no name for, and an empty inversion is what a third of the node
types already return. Only IntrinsicCondition has no safe default: Boolean.True
is the positive claim that an operation is total, and only the node's author
knows whether it holds, so it becomes protected abstract. Publishing the five
instead would drag two internal enums and 30 frozen enum values into the public
API for the same capability.

The document costs the alternative -- a closed hierarchy extended by data --
against that, and records why it is not the status quo either: Number.Real,
Rational, Complex and Variable are concrete and unsealed, and a subclass of one
compiles against the nupkg today and goes through Simplify and Solve intact.

The defaults were given to a node kind the kernel has never seen and run
through the pipelines EveryNodeSurvivesEveryPipelineTest uses. Sixteen of the
seventeen held, including like-term grouping and the provided machinery. The
one that did not is a twelfth obligation written nowhere: every node type
carries `ToString() => Stringize()`, and without it the record-synthesized
ToString recurses through Entity.PrintMembers until the stack guard fires.


Claude-Session: https://claude.ai/code/session_01Bjumi5K7fg8yx6UK1mZTQd

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

Author: Rafael-SOWNet