test(#397): enumerate the exact-sizing seam

[avrabe]

Second seam, after call_lowering in #403.

Why this one

can_size_exactly decides whether a type's size may reach a memory.copy length — the gate #393 turned on. Its structure is quietly fragile:

_ => true

A container variant that forgets to recurse falls into that arm and reports its contents as exactly sizeable. The resolver records a size built from the 4-byte fallback, and the copy runs short. That is #393 exactly, one level down, with nothing failing.

It is also the shape auto-Mythos flagged in my own #393 fix — a type escaping the check that was supposed to catch it.

Four tests plus a tripwire

• an unresolvable type must poison every container that can hold one, checked per variant so a failure names which;
• the mirror — a resolvable type must stay sizeable in every container. Without this, a can_size_exactly that simply returned false would satisfy the first test while disabling sizing entirely;
• nesting must not launder it (three deep, not one);
• leaves stay sizeable by construction.

The tripwire: variant_name matches every ComponentValType variant with no wildcard, so adding a variant breaks this test and forces a decision about whether the sizing seam must recurse into it. Given _ => true, silence is otherwise the default.

Control

Checked for reachability before running it this time. Removing the Option arm so it falls through to _ => true:

an_unresolvable_type_poisons_every_container          FAILED
  #397/#393: Option does not propagate un-sizeability
nesting_does_not_launder_an_unresolvable_type         FAILED
a_resolvable_type_stays_sizeable_in_every_container   ok      <- not a blanket-false
leaves_are_sizeable_by_construction                   ok

Naming the variant matters: the failure tells you which arm to fix, not just that something is wrong.

fmt clean, clippy -D warnings clean, 887 tests, exit 0.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WrmMqf1iuTS1UBEes5TmdC

[github-actions]

Mythos delta-pass required

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

meld-core/src/parser.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.

[github-actions]

LS-N verification gate

✅ 59/59 approved LS entries verified

	count
Passed (≥1 test, all green)	59
Failed (≥1 test failure)	0
Missing (no ls_*_NN_* test found)	0

_{Approved loss-scenarios.yaml entries are expected to have a
regression test named ls_<letter>_<num>_* (e.g. LS-A-11 →
ls_a_11_*). The gate runs each prefix via cargo test --lib --no-fail-fast and aggregates pass/fail/missing.}

Failed LS entries

(none)

Missing regression tests

(none)

_{Updated automatically by tools/post_verification_comment.py.
Source of truth: safety/stpa/loss-scenarios.yaml.}

[github-actions]

Mythos delta-pass (auto)

✅ NO FINDINGS across 1 Tier-5 file(s)

File	Verdict	Hypothesis
``	✅ NO FINDINGS	—

_{Auto-run via anthropics/claude-code-action@v1
(SHA-pinned) on the touched Tier-5 files, using the
maintainer's Max-plan OAuth token. See
.github/workflows/mythos-auto.yml and
scripts/mythos/discover.md.}