fix(lint): use the approved GcHeader accessor in the inline-slot-floor probe

[proggeramlug]

lint is red on main — scripts/addr_class_inventory.py exits 1 on one violation introduced by #7928:

crates/perry-runtime/src/object/tests.rs:646: [gcheader-cast] let gc = (obj as *const u8).sub(crate::gc::GC_HEADER_SIZE) as *const crate::gc::GcHeader;

Verified by running the checker in a clean worktree at origin/main (my own working tree is on another branch, which is why a local run initially passed and nearly had me dismiss the report).

Fix: use addr_class::try_read_gc_header, the accessor the checker's own error message names. It takes the object address and does the - GC_HEADER_SIZE arithmetic itself, behind its plausibility and small-slab checks — so the probe also stops assuming a header is present rather than asserting it.

No allowlist entry added. The ratchet exists so a fix deletes the violation; exempting it would be the thing the file's own README says not to do.

Why this matters beyond one line

A required context that is red means every merge bypasses it, and a genuinely new violation lands invisibly behind the stale one. That is CLAUDE.md's documented failure mode, and it was live: ~20 PRs merged into the collector today with admin bypass.

Validation

• python3 scripts/addr_class_inventory.py — passes (1056 files scanned, 284 allowlisted, 546 ratcheted sites); fails on origin/main.
• cargo check -p perry-runtime --lib --tests — 0 errors.
• cargo fmt --all -- --check — clean.

Found by the #7955/#7956 agent while validating #7974.