fix(runtime): grow large pre-sized arrays densely (rebased, from #9376)

[proggeramlug]

Lands #9376, rebased onto current main. Author's work preserved; the rebase resolution was done by a codex agent I ran on it, and I verified the result independently rather than accepting its report.

Why it needed a rebase. #9376's base predates #9370's reapply of #9297, so it targeted a shape of array/indexing.rs that no longer exists. It deleted DENSE_ARRAY_GAP_LIMIT, array_sparse_index_property_get and array_sparse_index_property_set, which have 11 references on current main, several inside the restored prototype-chain code.

The resolution keeps both behaviours rather than picking one: the three symbols move to the existing array/indexing_support.rs instead of being deleted, and dense frontier growth now runs only after the inherited-accessor and non-writable checks — so #9297/#9370's owner walk still happens first.

What I verified myself, not from the agent's summary:

• No baseline, allowlist, snapshot or ratchet file is touched by the diff — I checked the changed-file list explicitly, since making a gate pass that way is the obvious shortcut.
• The three symbols really are present in indexing_support.rs, i.e. moved rather than deleted.
• Gates re-run by me in that worktree: raw-handle 963 with no module ceiling raised and --no-raise-vs clean, plus file-size, root-holder, census, thread-local and unrooted-local.
• test_gap_9220_9221_array_proto_paths.ts byte-identical to the pinned Node 26.5.1 oracle using the agent's own build — that is the fixture that would catch the owner walk being lost.

The agent reported perry-runtime 2,929 passed / 0 failed and both fixtures matching node; the checks above are my independent confirmation of the parts that matter.