fix(cli): warn when meld fuse uses the multi-memory default (#172)

[avrabe]

Summary

Addresses #172 — meld fuse defaults to --memory multi, producing a fused module the canonical optimise → AOT chain can't consume.

A user running the happy path (meld fuse a b -o fused.wasm) gets a multi-memory core module. wasm-opt then rejects it without --enable-multimemory, and synth --cortex-m has no single flat address space for multiple memories. The wall is hit silently at the next tool.

What this PR does (#172 options 2 + 3)

• meld fuse prints a warning whenever --memory multi is in effect (including the default), naming the downstream implication and pointing at --memory shared --address-rebase.
• The --memory --help text documents the same.

$ meld fuse a.wasm b.wasm -o fused.wasm
warning: --memory multi produced a multi-memory module. wasm-opt needs
--enable-multimemory to consume it, and it has no single-address-space
(MCU) lowering. For a fused module the wasm-opt → synth chain accepts
directly, re-run with `--memory shared --address-rebase`.

Why not option 1 (flip the default)

#172's recommended option 1 is to default to --memory shared. That's deliberately not done here:

• Flipping the core memory default is high-blast-radius — it changes the output of every meld fuse invocation.
• shared is not an unambiguously safe default: meld-core documents it as broken when any component uses memory.grow ("one component's growth corrupts every other component's address space"), and the CLI currently labels it "legacy mode".
• The working downstream form per meld fuse defaults to --memory multi; fused module is rejected by wasm-opt and unusable for synth without extra flags #172 is shared --address-rebase — two flags, not just a default flip.

So the default flip is a separate decision for the issue owner. This PR makes the current default non-surprising in the meantime — the lowest-risk of #172's three suggested fixes, shippable now.

Tests

• test_cli_memory_default_is_multi — pins the default so a future flip is a deliberate edit
• test_cli_memory_shared_parses
• 7/7 meld-cli tests pass

🤖 Generated with Claude Code

[github-actions]

LS-N verification gate

✅ 19/19 approved LS entries verified

	count
Passed (≥1 test, all green)	19
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.}