Measure what a frame costs, so FR-DSP-2 is decided by numbers

`docs/display-and-extension.md` §2 fixes a decision rule in advance: if the
99th percentile of a frame sits inside 16 ms, tiled computation is rewritten
as a scheduling concern for export rather than built on the interactive path.
Nothing in the tree could answer that, so the rule had nothing to act on.

This is the instrument. It renders a 60 MP synthetic source through the real
`render_detailed` at three viewport sizes and four chain lengths, fit and
zoomed to 1:1, and reports nearest-rank percentiles rather than means — a
slider drag is judged by its worst frame.

Three things it does that a simpler timer would not:

- It separates the fused pass from the neighbourhood stage. "Every operation
  active" mixes one dispatch together with a chain of convolutions, and §2's
  question is about the first of those. `point` is every operation that
  contributes a fragment to the fused shader; `all` adds the four with
  kernels, and M3 times those alone by moving only a detail parameter so
  `render_detailed`'s colour reuse skips the fused dispatch. The reuse is
  reported rather than assumed — the `colour` column counts fused dispatches
  and must be zero for an M3 row to mean what it says.

- It times the CPU half separately. Composition runs per frame in
  `DevelopSession::render`, so it is inside the budget whether or not anyone
  has looked at it, and if shader assembly were the expensive half then no
  tile scheduler could help.

- It builds the "every operation" chain from `EditGraph::capabilities` rather
  than from a list, so declaring a new node does not quietly turn that row
  into a shorter chain wearing a longer chain's label.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/traceability.md

@@ -9,7 +9,7 @@ Denominators are parsed from [`requirements.md`](requirements.md) at run time, n
 
 | Metric | Value |
 |---|---|
-| Source files scanned | 254 |
+| Source files scanned | 255 |
 | TRACES tags found | 732 |
 | Requirements defined | 177 |
 | Requirements covered | 98 |