v0.3.0 — Concurrency: Channels, Task Handles, Auto-Arc, Mixed CPU+I/O Overlap

Commit range: v0.3.0-m7..v0.3.0 (M3g + M4 content only — M3f and M3d, though present in this
git range because they merged after the v0.3.0-m6/v0.3.0-m7 tags were cut, are already
covered above and are NOT re-described here).

What ships

This is the final v0.3.0 release: the milestone where Yinz code genuinely runs
concurrently, safely, on one unified suspension substrate. It closes the last gap in
auto-parallelization (mixed CPU+I/O groups), and adds the concurrency-communication
surface — bounded channels, task handles, safe cross-thread sharing, and false-sharing
protection — on top of the poll-based substrate M3a/M3b/M3d already proved out. The
may-block suspension question is answered in exactly one authoritative place, threaded
into every consumer, closing a class of twin-computation-drift bug that had silently
miscompiled four prior milestones (M3a/M3d/M3e/M3g).

M3g — mixed CPU+I/O overlap

• A function body mixing one heavy CPU call and one I/O (wait) call as independent
  operations now runs them concurrently — CPU on a worker core, I/O suspended — instead
  of sequentially. This was the last gap in "all independent operations
  auto-parallelize": M3b covered pure I/O overlap, M3d covered pure-CPU overlap; M3g
  fuses the two poll paths (one shared continuation re-drives both the CPU join-poll and
  the I/O inline-poll on every resume) so a mixed group overlaps too, without
  deadlocking — the failure mode a verify-first pass on M3d had proven genuinely
  milestone-sized, not avoidance.
• Output remains byte-identical to --no-auto-parallel for every fixture; the flag
  self-gates the fusion exactly as it gates pure-CPU and pure-I/O promotion.

M4 — channels, task handles, auto-Arc, false-sharing padding

• channel<T>() bounded channels — construction, send()/recv(), default capacity
  64 (channel<T>(N) to override; no unbounded constructor by design). send() on a
  full channel suspends the caller through the same poll-yield substrate as wait —
  "a suspended producer is backpressure working correctly, not a deadlock," documented
  as such in docs/reference/REF-concurrency.md.
  Channel operations join the unified may-block source like any other suspension point.
• Background task handles — let h = background worker(...) yields a handle
  supporting .send() into the running task and repeated .receive(), covering both
  message replies from a long-running task and a plain suspending function's own
  completion value. A collected result's ok-value is copied to a handle-owned buffer
  before the frame frees (compile-time spawn-form-keyed — no runtime "was this
  collected?" tracking anywhere); the bare fire-and-forget spawn path is unchanged.
• Cross-thread auto-Arc — boundary exactness. The share/lend-across-background
  reject is now exact in both directions, including previously-silent unresolvable/
  non-ident-callee call sites. The acquire-release runtime substrate ships; the codegen
  EMISSION that would make cross-thread sharing atomic is deferred to v0.4+ (a
  documented, tracked deferral — today's cross-thread read access is already safe via
  the existing independent-copy path).
• False-sharing auto-padding. Shapes with fields touched by different background
  tasks get automatic 64-byte cache-line padding — codegen-only, no source change
  required. The cross-thread-fields-not-padded Tier 3 lint fires when the compiler
  can't safely auto-pad (cross-module-visible layout). This is the first layout
  transform gated by --no-auto-parallel, and the gating is proven, not assumed.
• [[lint_rule]] registry mechanism — built from scratch (schema, parser, build.rs
  constants, LSP seam), generic enough for future lint domains (M5's auto-SoA lint
  reuses it with zero rework). Carries cross-thread-fields-not-padded and
  prefer-yielding-sleep (suggests the yielding wait sleep(ms) over the
  thread-parking sleepBlocking(ms) in schedulable programs).
• Teaching surface — a channel_capacity muted inlay hint (shows the default 64,
  click-to-make-explicit); auto_arc is registered with its full cautionary hover text
  (fires once the v0.4+ emission lands); docs/reference/REF-concurrency.md gained the
  full channel/handle-form user spec including the backpressure teaching text; the VSCode
  extension bumped to 0.3.0.

Tests

The R6 may-block unification parity test (build-blocking — the fifth touch of the
twin-derivation class, now closed with one authoritative source); the R5 composed
deadlock-safety hostile fixture (child suspends on a full channel while the parent
polls .receive() — proven safe through the real compiler, not asserted); R1/R2
hostile fixtures (never-received channels, pool exhaustion); the R8 collection matrix
including the composed R5×R8 cell (a collected background child that both suspends on
send() and is collected at completion); the R3 cross-thread boundary-exactness matrix;
a full --no-auto-parallel cross-impl sweep across every new fixture, including the
false-sharing padding transform and the composed-suspension case, byte-identical in
both modes. Full workspace suite, clippy, and fmt green.