Retire the reg and Recurse names, and pin the out-of-block read rule

[dpmills]

The mutability subsystem renamed its concept to "mutable variable", but the identifiers carrying it still said register — the fresh name was __reg, a key's history binding was reg_k, locals were reg_read / reg_vtys — and __reg reaches symbolic output and test assertions, so the old word outlived the rename exactly where a reader meets it. This retires it (the record is __hist, its fields hist_k, and the prose calls it a history record) along with a second name in the same state: Recurse, the removed recurrence operator behind an induction accumulator, still cited in twenty-five comments across thirteen files as though it were something to go look at. Each of those names what it now means, since the old one stood for the induction-store engine in some places and a loop's own cycle in others.

The out-of-block read is a rule, not a caution

mutability.md presented the non-atomic pre-check if balance > 0: with begin(): balance -= req as compilable but unwise, and asked for a lint. The guard reads a Txn variable outside a block, which lowering rejects and always will, so the shape does not compile and there is nothing to lint; the spec's reads bullet is marked [Decided] to match. The rule had no test where it is easiest to lose — following a Mut(_, Txn) parameter into a callee — and now has one.

Prose

mutability.md also drops the voice its own rules call out: stress-italicised copulas and negations throughout, deliberately seven times, Footgun as a heading, and strict wall for the strict typecheck, a metaphor already colliding with two other mechanisms.

[dpmills]

This change is part of the following stack:

• Feed both writers' decision bodies through tiles, retiring WriterBuffer #93
  • Close a commit store's keys individually, so an await waits only for its own variable's writers #102
    • Retire the reg and Recurse names, and pin the out-of-block read rule #106 ◀
      • Add an end-to-end walkthrough of mutability, from CHL source to operators #97

_{Change managed by git-spice.}