docs: MAE has no garbage collector — stop claiming one in three places

[cuttlefisch]

CLAUDE.md principle #1 said we use "a purpose-designed concurrent GC for the
Scheme runtime". We do not, and nothing like it is built.

Verified against the tree

• The Scheme heap is Rc<T> refcounted with no tracing collector
  (crates/scheme/src/value.rs, "Stage 1"). A Trace trait exists as
  groundwork; nothing walks it.
• collections_count is declared (vm.rs:214), copied to the editor
  (runtime.rs:944), and reported by (gc-stats), gc_stats_alist and the
  introspect MCP tool — but nothing increments it. Every write to the field
  was checked. (gc-stats)'s collections is permanently 0.
• (gc-collect!) is a no-op whose own doc string said it "increments counter in
  Rc stage", and whose body comment said "the VM increments
  gc_stats.collections_count when this runs". Neither was ever true.

Why three places matter

The same false claim sat in the file that primes every AI session, in a
registered Scheme doc string any user can call up, and in the comment a
maintainer would read immediately before touching the code. Fixing one and
leaving the others would just relocate the misinformation.

Principle #1 now also spells out what a reader must not be misled about: Rc is
!Send, so the runtime is single-threaded by construction and "no GIL, ever" —
while true — is vacuous at Stage 1, since there is no shared multi-threaded
interpreter for a lock to guard. Refcounting also leaks reference cycles, and
code_pool is append-only.

Deliberately no test

A test asserting collections == 0 would pin a no-op and obstruct the collector
we actually want someone to build. The @ai-caution: [docs-drift] marker at the
site carries the warning instead — including the specific wrong fix, which is
incrementing the counter to match the docs. A counter that rises without a
collection is a worse lie than one that stays at zero, because it reads as
evidence that collection is happening.

Amended in the open per principle #17, which asks for exactly this rather than
letting an aspirational claim stand in the present tense.

No overlap with any PR currently open.

🤖 Generated with Claude Code