Validate out-of-order tables incrementally while parsing

[gaoflow]

Fixes #479.

Parsing n dotted keys (or table headers) that share a prefix is super-cubic: every append to an out-of-order key triggers OutOfOrderTableProxy.validate, which re-merges — and deep-copies — every earlier fragment into a fresh temp container, and the membership probe in append() rebuilds the full proxy each time as well. Profiling the issue's payload shows ~13M deepcopy calls for 60 keys.

Three changes, all confined to parse time (_parsed containers):

• _validate_out_of_order_table keeps a per-key cache of (fragments validated so far, temp container) and resumes from there, so each fragment is merged and deep-copied exactly once. Fragments are only appended during parsing; any other mutation (remove, _remove_at, _replace_at) clears the cache, and a validation failure drops the entry so a caught-and-retried error can't resume from a partially merged container.
• __contains__ skips rebuilding the proxy when the cache shows exactly the current fragments already validated clean.
• The no-key validation sweep iterates a maintained set of out-of-order keys instead of every key in the map (the per-key path still re-checks isinstance(idx, tuple), so stale entries are harmless).

Benchmark (the issue's repro):

keys	before	after
40	1.3 s	5.5 ms
80	15.6 s	10 ms
160	219 s	19 ms
1000	—	0.15 s

Error behavior is unchanged: collisions across fragments still raise at parse time (added tests), and the existing suite passes unmodified.