Fix LT-22575: Deleting a duplicated phonological rule deletes objects the original still uses

[jasonleenaylor]

Problem

LT-22575: after duplicating a phonological rule (LT-21904, #342), deleting either copy corrupts the survivor.

A PhRegularRule's alpha-variable PhFeatureConstraints and its sequence/iteration environment members are owned by the PhPhonData pools (FeatConstraintsOS, ContextsOS) and only referenced by the rule's contexts. CopyObject correctly shares references to objects outside the copied subtree, so a duplicate references the same pooled objects as the original. The deletion side effects, written long before duplication existed, removed those pooled objects from their owning pools unconditionally — assuming the dying rule was the sole referrer. Deleting one copy therefore deleted the other copy's feature constraints, and could cascade through a shared sequence environment to null the survivor's LeftContextOA/RightContextOA, silently wiping its environment.

Fix

Guard the four GC sites with the model's incoming-reference bookkeeping (ICmObject.ReferringObjects, IPhContextOrVar.Rule) — no new mechanism, no CopyObject changes, no object copies:

• PhRegularRule.OnBeforeObjectDeleted: GC a pooled constraint only when every referring context resolves to the dying rule.
• PhRegularRule.RemoveObjectSideEffectsInternal (StrucDesc edit path): GC only when the removed context is the constraint's last referrer anywhere, not just within the editing rule. The LT-21729 remove-ref-first ordering is preserved.
• PhSequenceContext/PhIterationContext.OnBeforeObjectDeleted: GC a pooled member only when the dying container is its last referrer.

GC still completes: when the last referring rule is deleted, the pooled objects are removed with it (each test asserts the pool empties). This matches the codebase's established referrer-aware precedent (MoMorphSynAnalysis.CanDelete counts referrers before allowing deletion).

Tests

Four tests in LingTests.cs, each red before the fix and green after:

• DuplicateRegularRule_DeletingCloneDoesNotDeleteOriginalsFeatureConstraints — the reported repro
• DuplicateRegularRule_SharedConstraintSurvivesUntilLastRuleDeleted — constraint shared (not copied), survives first deletion, GCed on last
• DuplicateRegularRule_RemovingContextKeepsConstraintUsedByClone — the edit path
• DuplicateRegularRule_DeletingCloneKeepsSharedEnvironmentMembers — the environment-wipe cascade

Full SIL.LCModel.Tests suite (net8.0): 1701 passed, 0 failed.

Notes for reviewers

• PhMetathesisRule needs no change (it has no constraint GC).
• MoAffixProcess shares the same FeatureConstraints structure but has never GCed constraints on deletion; that pre-existing leak-by-omission is out of scope.
• GetFeatureConstraintsExcept is no longer used by the edit-path GC but remains public API and backs the FeatureConstraints virtual property.

🤖 Generated with Claude Code

---

This change is 

[github-actions]

LCM Tests

    16 files  ± 0      16 suites  ±0   3m 13s ⏱️ +16s
 2 867 tests + 4   2 847 ✅ + 4   20 💤 ±0  0 ❌ ±0 
11 416 runs  +16  11 248 ✅ +16  168 💤 ±0  0 ❌ ±0 

Results for commit 6061912. ± Comparison against base commit a581a64.