fix(p3-async): mixed-mode stackful, type classification, stream-write over-count

[avrabe]

Summary

Second fix from the post-v0.8.0 Mythos delta-pass sweep. Three independent
defects in `meld-core/src/p3_async.rs`, all silent (no trap, no validator
catch).

The bugs

#	LS	Site	Effect
1	LS-A-12	`uses_stackful_lift()` line 952	Mixed-mode component (one callback lift + one stackful lift) returns `false` because the derived predicate `uses_async_lift && !uses_callback_lift` collapses
2	LS-A-13	`StreamWriteResult::decode` line 813	`written >= requested` folds `n > data_len` into `Complete { written: n }` — caller advances cursor past source buffer
3	LS-A-14	`detect_features` line 964	`desc.contains("stream")` matches `future<stream>` → type lands in `stream_types`

Fixes

1. LS-A-12 — New independent field `uses_stackful_lift_internal: bool`
   set per-lift in `detect_features`. `uses_callback_lift` (any lift has
   callback) and `uses_stackful_lift_internal` (any lift is stackful) are
   now independent. The dispatcher in `adapter/fact.rs` already routes
   per-site based on the merged module's `[callback]` companion,
   so this never reached the emitter — but downstream consumers following
   the public API would misroute. Bug shipped in feat(p3-async): stackful lifting ABI foundation (#140 phase 1) #146 / v0.8.0.

2. LS-A-13 — Split `>=` check into three explicit arms:
   `==` → `Complete`, `<` → `Partial`, `>` → `Unknown(ret)`.

3. LS-A-14 — Classify on the root constructor (prefix up to first `<`),
   not on substring containment.

Tests (4 new)

• `stackful_lift_is_existential_over_lifts` — replaces the old
  single-lift truth-table test; covers the mixed-mode case
• `ls_a_12_mixed_mode_component_reports_stackful` — end-to-end
  `detect_features` with both lift kinds
• `ls_a_13_stream_write_over_count_is_unknown_not_complete`
• `ls_a_14_future_of_stream_classifies_as_future`

Tier-5 gate

This PR touches `p3_async.rs` (Tier-5). The Mythos gate will fire and
request the `mythos-pass-done` label. The pass IS the work in this PR.

Test plan

• `cargo test -p meld-core --lib` — 211 pass (208 prior + 3 new SR-32-cluster + 0 others moved)
• `cargo clippy --all-targets -- -D warnings` — clean
• `cargo fmt --check` — clean
• YAML lint on safety/stpa/loss-scenarios.yaml
• CI green on smithy
• mythos-pass-done label applied

Note on overlap with #152

This PR is independent of #152 (extended-const fix). Both add entries
to `safety/stpa/loss-scenarios.yaml` and `CHANGELOG.md` Fixed section.
Whichever merges first wins; the other will need a small textual rebase
(additive only, no real semantic conflict).

Refs

• LS-A-12 (UCA-A-3, H-1 / H-3)
• LS-A-13 (UCA-A-5, H-1 / H-2)
• LS-A-14 (UCA-P-10, H-3 / H-4)
• Discovered by post-v0.8.0 Mythos delta-pass on `p3_async.rs`

🤖 Generated with Claude Code

[avrabe]

Mythos delta-pass evidence

Tier-5 file touched: meld-core/src/p3_async.rs.

A fresh agent ran scripts/mythos/discover.md on the file in the post-v0.8.0 sweep, returning 3 findings (LS-A-12 / LS-A-13 / LS-A-14). All three have concrete PoCs that reproduce on current code; validation done via reproducing the PoC tests against pre-fix code (commits before f7dc651).

The agent also verified-and-rejected 3 priors (PAYLOAD_SHIFT docstring vs emitter, missing intrinsic variants, name typos) — those checks remain in the agent transcript and are NOT defects.

Adjacent findings not in this PR:

• None — this PR fixes all 3 strong findings from the p3_async.rs scan.

[github-actions]

Mythos delta-pass required

This PR modifies one or more Tier-5 source files (per
scripts/mythos/rank.md):

meld-core/src/p3_async.rs

Before merge, run the Mythos discover protocol on the
modified Tier-5 files:

1. Follow scripts/mythos/discover.md
   — one fresh agent session per touched Tier-5 file.
2. For each finding, the agent must produce both a Kani
   harness and a failing PoC test (per the protocol's
   "if you cannot produce both, do not report" rule).
3. Attach a comment on this PR with either the findings
   (formatted per discover.md's output schema) or
   NO FINDINGS.
4. Add the mythos-pass-done label to this PR.

Why this gate exists: LS-A-10
(CABI alignment padding in async-lift retptr writeback) was
found by the v0.8.0 pre-release Mythos pass — but it had
lived in the callback emitter since #128, across six
releases. A PR-time gate would have caught it at review
time instead of at the release boundary.

The gate check on this PR will pass once the label is
applied.