collapse_up merges constraint-carrying Up instructions, dropping intermediate last-child checks

[zharinov]

Problem

collapse_up merges two directly-adjacent constraint-carrying Ups (e.g. two UpSkipTrivia(1)) into one UpSkipTrivia(2). That is wrong.

The VM's go_up runs the last-child check once, before ascending N levels. So a merged Up(2) only checks last-child-ness at the innermost level. The outer level's check is dropped.

Nested trailing anchors silently lose all but the innermost assertion:

• soft . over-matches
• strict .! mis-matches

# soft: Want no match — (object) is not the last named child of (array), `b` follows
# Got:  {"p":{"kind":"pair",...}}
cargo run -p plotnik -- run -l javascript -s '[{a:1}, b]' -q 'Q = (program (expression_statement (array (object (pair) @p .) .)))'

# strict: Want no match — (return_statement) is not the absolute-last child of the block, `}` follows
# Got:  {"id":{"kind":"identifier","text":"x",...}}
cargo run -p plotnik -- run -l javascript -s 'function f(){ return x }' -q 'Q = (program (function_declaration body: (statement_block (return_statement (identifier) @id .!) .!)))'

dump shows the two Up(1)s collapsed to ─•┘² (UpSkipTrivia(2)) / !─┘² (UpExact(2)).

Cause

compile/collapse_up.rs:151-160 same_up_mode allows merging the three constraint-carrying modes (UpSkipTrivia/UpSkipExtras/UpExact). The merge gate is collapse_up.rs:85.

crates/plotnik-vm/src/engine/cursor.rs:140-189 go_up runs the lastness check once (lines 142-181), then ascends levels times in the loop (184-188). Each constraint mode means "assert last-child at every level," but the merged instruction only checks the innermost one.

Only plain Up(n) (UpMode::Any, no per-level constraint) is safe to coalesce.

docs/tree-navigation.md:264 already documents this exact merge as unsound, but the code permits it and compile/collapse_up_tests.rs:80-187 assert it should happen.

Fix

Restrict collapse_up to plain Nav::Up only:

• Keep the (Up, Up) arm in same_up_mode, reject the three constraint-carrying modes.
• Update the module doc (collapse_up.rs:1-9) and docs/tree-navigation.md:240.
• Invert or remove the three tests that codify the unsound merge: collapse_up_tests.rs:80-106 (skip_trivia), :109-135 (skip_extras), :160-187 (exact).

Acceptance

• Both repros return no match.
• conformance nested_trailing_soft_anchor_drops_outer_last_child_check and nested_trailing_strict_anchor_drops_outer_last_child_check flip to <no match>; nested_trailing_soft_anchor_matches_when_truly_last still matches.

Related

The fix must invert the three collapse_up_tests.rs cases that assert the unsound merge — they are part of the bug, not a regression guard.