ctrl#1533 phase 2: swap Value map/list host carriers to im-rc HAMT/RRB (persistent updates, copied-counters collapse to 0)

[briansrls]

Phase 2 of the persistent-structures plan (ctrl #1533; phases 0/1 = #4653, #4660, #4666): swap the v2 interpreter's host carriers behind the Value surface to the carriers phase 1 declared. No DSL-observable semantic change.

What changed

• Value::Map now carries Rc<im_rc::HashMap> (HAMT — matches map_carrier ^hash_array_mapped_trie), Value::List carries Rc<im_rc::Vector> (RRB — matches list_carrier ^relaxed_radix_balanced_tree, O(log n) concat). im-rc over rpds because it provides BOTH declared carriers (rpds::Vector has no concatenation) and is Rc-backed, matching the interpreter's Rc discipline (rationale comment in Cargo.toml).
• The Rc wrapper stays: value_rc_identity (pure-call memo keys) and the retained-accounting sharing dedup key off Rc::as_ptr. M-C law 1 stays one-way — pinned by a host-side probe (two runs build pointer-distinct, ==-equal values).
• map_insert/merge and list_push/concat are now persistent updates; the *_entries_copied / *_items_copied counters count nothing there anymore. Only the FreeMonoid flatten chokepoint (ctrl#1476 B1) still materializes items — exactly the L1/L2 split ctrl chore(r3-debt-paydown): velocity-tripwire script + first 2026-05-02 measurement (FIRED 25.33:1) #1534 documents.
• B1 alias transparency preserved: native List and Empty/Cons chains route through one value_to_list_carrier bridge (no per-representation operand arms; list_free_monoid_chokepoint_test still green).

Receipt (the phase-2 number)

GUNBC_INTERP_STATS=1 claim_batch over the 9 value_carrier_laws witnesses:

map_insert    18 calls    0 entries copied   (phase 0: copied ≈ n(n−1)/2, triangular)
list_concat    6 calls    0 entries copied
fm_flatten    49 calls   49 items materialized   (the remaining, intended cost)

interp_stats_test re-pins the new semantics: the workloads that copied 6/9/3 entries on main now copy 0 (tests renamed to say so).

New witnesses

• value_carrier_laws.dag: list/map prior-version persistence (Driscoll et al. — derived version leaves the base valid; the failure class a carrier swap can introduce is in-place mutation through a shared handle). 11 claims in the file, all PASS.
• value_carrier_swap_test.rs: host-side at-scale probes (n=500, past im-rc's 64-element inline chunks so HAMT/RRB tree paths actually run): insert-order-independent equality, overwrite path-independence, prior-version validity, identity non-observability.

Gates run

• cargo test -p v2-compiler-tests: 27→24 failures vs baseline — all 24 are the pre-existing pipeline::* failures (verified identical set on the unmodified branch); the 4 new probes pass.
• stage0 freshness: regen_stage0 --verify → "committed stage0 matches fresh self-compile".
• parity (release, --exact): PASS.
• cargo clippy --all-targets -- -D warnings, cargo fmt --all --check: clean.
• 9/9 carrier-law witnesses PASS by execution (claim_batch --claim-run).

Phase 3 (interning + positional records, M-D) and phase 4 (re-measure) follow separately.

🤖 Generated with Claude Code

[briansrls]

Re claude/composer review nit (redundant inner parens in ~16 list_value((x)) sites, e.g. v2_interpreter.rs:1183,1359,2378): confirmed real — leftover from a scripted edit during the carrier swap; cosmetic only, no behavioral effect (and the reviewer marked it non-blocking).

Deliberately NOT pushing a cleanup commit to this PR: it's merge-ready on this HEAD (2 approvals, CI green) and a cosmetics-only push would stale both approvals while the operator is merging manually. The paren cleanup is queued as the first commit of the ctrl#1533 phase-3 PR, which touches the same file.

— sent from neat-ibex-297