Make Entity's adhoc members list lazy

[auduchinok]

Every Entity carried two objects most entities never use.

tcaug_adhoc_list — TyconAugmentation.Create allocated an empty ResizeArray per entity, and
exactly one site ever adds to it (PublishValueDefnMaybeInclCompilerGenerated). Nothing is added for an
imported type: NewILTycon closes the augmentation and publishes no ad-hoc members. It is now allocated at
that single site behind AddAdhocMember, with AdhocMembers for the two readers, and unpickling an empty
list stores nothing.

entity_il_repr_cache — a cache cell only IlxGen reads. An analysis-only host allocates one per
entity and never touches it. CompiledReprCache now creates it on first request, so the member's
signature is unchanged and no caller can observe a null.

No FCS public API change; the internal record fields become nullable, tcaug_adhoc_list becomes mutable,
and TyconAugmentation gains AddAdhocMember / AdhocMembers.

One project held open (mean of 3 fresh processes)

Project	Before	After	Retained
consoleapp	33.84 MB	33.76 MB	-0.08 MB (-0.24%)
Oxpecker	72.83 MB	72.52 MB	-0.31 MB (-0.43%)
IcedTasks	50.14 MB	50.03 MB	-0.11 MB (-0.23%)
FsToolkit	75.39 MB	75.23 MB	-0.16 MB (-0.21%)
Fantomas.Benchmarks	64.77 MB	64.71 MB	-0.06 MB (-0.09%)
Prime	108.37 MB	108.23 MB	-0.14 MB (not established)
Fantomas.Core	109.50 MB	109.26 MB	-0.24 MB (not established)
Fantomas.Core.Tests	141.95 MB	141.74 MB	-0.21 MB (not established)
FSharp.Common	297.67 MB	296.35 MB	-1.32 MB (-0.44%)
fcs	1273.30 MB	1273.31 MB	+0.01 MB (not established)

A whole solution held open at once (mean of 2 passes)

Solution	Before	After	Retained
resharper	1746.27 MB	1737.29 MB	-8.98 MB (-0.51%)
fcsrepo	2557.91 MB	2555.86 MB	-2.05 MB (-0.08%)
fantomas	690.18 MB	689.28 MB	-0.90 MB (-0.13%)
oxpecker	254.06 MB	252.19 MB	-1.87 MB (-0.74%)
fstoolkit	238.00 MB	237.82 MB	-0.18 MB (not established)
icedtasks	212.48 MB	211.74 MB	-0.74 MB (-0.35%)
prime	231.86 MB	231.77 MB	-0.09 MB (not established)
consoleapp	33.82 MB	33.75 MB	-0.07 MB (-0.22%)

What it removes

gcdump of one held FSharp.Common analysis:

	before	after	never used
cache<CompiledTypeRepr>	26,465 (0.61 MB)	2,873 (0.07 MB)	89%
List<bool * ValRef>	26,721 (0.82 MB)	1,413 (0.04 MB)	95%

1.32 MB of objects against 1.32 MB measured. The right-hand column is the argument for the change: 95% of
augmentations never receive an ad-hoc member and 89% of entities are never asked for a compiled
representation, because this host only analyses.

[github-actions]

❗ Release notes required

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---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
`src/Compiler`	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

[T-Gro]

The invariant that makes this safe reads cleanly: tcaug_adhoc_list is now always either null or non-empty — Create starts null, AddAdhocMember allocates on first write, unpickling an empty list stores null, and remapping maps null to null. Nothing downstream consumes the list except pickling (through AdhocMembers) and remapping; IlxGen sorts off tcaug_adhoc / MembersOfFSharpTyconSorted, never the list, so the lazily-allocated field has no hot-path reader to trip over.

Lazy CompiledReprCache is equally benign under concurrency: the field is a reference written atomically and cached was already non-atomic, so a racing first-touch costs at most one redundant, deterministic recompute — and the entities most likely to be shared across parallel-checking threads (imported ones) still allocate eagerly in u_entity_spec_data, so the window is tiny.

Built Debug clean, 0 warnings with nullness checking on. Measurement-backed and tightly scoped — lovely change.

One optional follow-up: u_entity_spec_data still does entity_il_repr_cache = newCache() for every unpickled entity, so imported types keep paying for a cache an analysis-only host never reads. Since CompiledReprCache now tolerates null, that site could be null too and reclaim the imported-entity share as well.