[EXTRA] Simplify StructuralMap engine policy

[tqchen]

[EXTRA] Simplify StructuralMap engine policy

StructuralMap needs identity state only to keep variables consistent when
descent rewrites a definition. Callback results are separate: callbacks run
at every occurrence in their selected pre- or post-descent position.

Make default C++ descent the sole owner of FreeVar and DAG remapping, with
definitions binding rewritten results and unchanged simple definitions left
absent. Keep the remap ABI stable and use an owned raw-pointer-keyed store for
efficient identity lookups.

Adopt whole raw TVMFFIAny results by value while leaving destructor-visible
slot moves pointer-based. Initialize owning Any storage member-wise so GCC
retains scalar state and avoids an overlapping narrow-store/wide-reload stall.

Update existing C++ and Python expectations in place and document
StructuralMap as post(D(pre)) alongside StructuralWalk's stateless tree
traversal. Rust remains unchanged for a follow-up.

[tqchen]

Impact of the recent StructuralMap speed improvements (#768 and this PR)

Preliminary numbers from a minimal benchmark (a mini-TIR built from tvm-ffi types: a split/fuse expression tree and SeqStmt blocks), one pinned core, 21 interleaved processes per state, median of process medians, ns/node, with hooks following this PR's map policy and writing fields back only when changed. OLD is the engine before #768 (e74e58f), UC is current main with #768 (UnchangedOr results), NEW is this PR; deltas against OLD. never is a callback that matches nothing; retained hands the engine a shared root, moved the sole reference.

x86 (AMD EPYC 7413)

arm	fixture	ownership	OLD (ns/node)	UC (#768) (ns/node)	NEW (this PR) (ns/node)
never	split-fuse-distinct	retained	25.01	20.49 (-18.1%)	11.31 (-54.8%)
never	seq-256	retained	28.35	22.08 (-22.1%)	14.67 (-48.3%)
subst	split-fuse-distinct	retained	48.20	45.98 (-4.6%)	34.65 (-28.1%)
subst	split-fuse-distinct	moved	40.46	34.99 (-13.5%)	22.15 (-45.3%)
swap	seq-256	retained	36.98	32.81 (-11.3%)	24.05 (-35.0%)
swap	seq-256	moved	34.37	32.41 (-5.7%)	24.21 (-29.6%)

aarch64 (Neoverse V2)

arm	fixture	ownership	OLD (ns/node)	UC (#768) (ns/node)	NEW (this PR) (ns/node)
never	split-fuse-distinct	retained	47.10	25.75 (-45.3%)	8.84 (-81.2%)
never	seq-256	retained	53.55	36.05 (-32.7%)	14.80 (-72.4%)
subst	split-fuse-distinct	retained	83.04	75.44 (-9.2%)	47.34 (-43.0%)
subst	split-fuse-distinct	moved	72.24	50.08 (-30.7%)	25.34 (-64.9%)
swap	seq-256	retained	67.97	48.53 (-28.6%)	26.11 (-61.6%)
swap	seq-256	moved	61.02	45.94 (-24.7%)	21.37 (-65.0%)