avoid quadratic multiline-string context scan in is_line_short_enough

[sahvx655-wq]

Description

While profiling Black on a generated config module I noticed formatting hung for several seconds on a single dict literal whose values were all triple-quoted strings. A flame graph put ~80% of the wall time in is_line_short_enough, specifically the while str(ctx) in line_str loop that walks the multiline string up through its enclosing AST nodes. Each step renders the whole ancestor subtree to a string and substring-searches the line, so when the topmost ancestor is the entire collection that render is O(N), and because is_line_short_enough runs once per candidate split line during line generation the whole pass is O(N²). A dict with 1600 triple-quoted values took ~2.2s; the cost grew quadratically with the number of members.

The walk only needs the ancestors that are wholly present on the current line, and since a line is a contiguous run of leaves an ancestor qualifies exactly when its first and last leaves both belong to the line. Checking two leaf ids against a set built once is O(depth) per step and bails out immediately for ancestors that aren't fully on the line, so the render-and-search disappears and the pass becomes linear (the 1600-value case drops to ~0.23s). Keeping the check here rather than at the call sites means every caller of is_line_short_enough benefits without having to special-case multiline strings. Output is unchanged: byte-identical across the test suite, the Black and stdlib sources in stable/preview, and several thousand fuzzed multiline-string structures.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

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diff-shades results comparing this PR (33fb575) to main (b74b230):

--preview style: no changes

--stable style: no changes

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