feat(diagnostics): real v8 heap snapshot from GC heap walk + inspector/repl honesty (#4916)

[proggeramlug]

Closes #4916.

What

The diagnostics audit had one SILENT-LIE (empty-but-valid heap snapshots) and two PARTIALs (inspector, repl). This PR makes the snapshot real and makes the rest honest.

Real heap snapshot (gc/heap_snapshot.rs, new)

v8.getHeapSnapshot() / v8.writeHeapSnapshot() now emit an actual object graph built from Perry's GC heap walk, in Node v26 / V8 13.x snapshot format:

• Nodes: every arena + malloc GC allocation on the calling thread after a forced full collection — real self_size, type mapped from GC_TYPE_* (objects/arrays/strings/closures/bigints/native), class instances named via the class registry, string nodes carry their content (truncated at 128 chars).
• Edges: enumerated with the same visit_gc_rewrite_slots walk the marker traces, so the edge set is definitionally what the GC sees. Object inline fields resolve to named property edges through the keys array, array elements get element edges, weak slots get weak edges, everything else (captures, Map/Set side tables, overflow) is an internal edge.
• Reachability: a synthetic (GC roots) node points at every zero-indegree node plus any pure cycles, so DevTools (which drops unreachable nodes) shows the whole dump.
• Stub flags + first-call warnings on both APIs removed; stub_inventory.rs now pins them must-not-be-stub.

Validated by loading Perry-produced snapshots in Node and checking: 0 invalid edge targets, 100% root reachability, named property edges present, class-instance nodes present (object:Widget ×5), marker strings/keys in the string table. test-files/test_gap_v8_heap_snapshot_4916.ts byte-matches node --experimental-strip-types (v26.3).

Found & sidestepped: pre-existing gc() liveness bug (#4977)

While validating, live top-level locals vanished from dumps and read back as garbage with plain explicit gc() too — in the default auto stack-scan mode a full collect skips native-stack roots and reclaims live objects (const k={nested:{deep:"s"}}; gc(); corrupts k.nested.deep; fine under PERRY_CONSERVATIVE_STACK_SCAN=full or PERRY_GEN_GC=0). Filed as #4977 with the bisection. The snapshot's forced collection runs under the existing per-thread set_conservative_stack_scan_override(Full) so the diagnostics API cannot inherit the corruption.

Heap stats provenance (acceptance item 2)

getHeapStatistics / getHeapSpaceStatistics / getHeapCodeStatistics / GCProfiler.stop get manifest stub_notes that state exactly which fields are Perry-arena numbers, which are RSS, and which are fixed/zero — rendered into reference.md / perry.d.ts (regenerated here).

Inspector (acceptance item 3)

• inspector.url() → always undefined. It used to fabricate ws://host:port/uuid for an endpoint that never existed.
• open() no longer prints the fake Debugger listening on ws://… banner; first-call stub warn; keeps ERR_INSPECTOR_ALREADY_ACTIVATED/close() semantics.
• waitForDebugger() stub-warns (it returns immediately — blocking would hang forever since nothing can attach).
• Session.post(): the complete supported subset (Runtime.enable + canned Runtime.evaluate) is documented at run_command and in the manifest; unsupported methods keep the spec'd -32601.

REPL verified (acceptance item 4)

Confirmed the suspicion: repl.start() builds the server object, but nothing ever reads the input stream and .write() routes through eval_simple_expression (numeric literals, context lookups, one +). Flagged stub: true with a first-call warning — a real eval loop needs an embedded interpreter, out of scope for AOT Perry.

Tests

• RUST_TEST_THREADS=1 cargo test --release -p perry-runtime — 1021/1022; the one failure (date::tests::test_full_year_setters_revive_invalid_date_only) fails identically on a clean origin/main tree (pre-existing, macOS-local).
• perry-api-manifest (incl. updated stub_inventory drift guard) and perry-codegen manifest_consistency — green.
• Gap tests test_gap_node_v8_3137plus, test_gap_v8_2_3680plus, test_gap_v8_heap_snapshot_4916 — byte-identical to node v26.3.
• Docs regenerated via ./scripts/regen_api_docs.sh (api-docs-drift clean).

Per contributor convention: no version bump / changelog — maintainer folds metadata at merge.