fix(dataviz): honour start_at, the layout box, and degenerate scales

[LeadcodeDev]

Refs #142 — Dataviz workstream. Eight confirmed findings, plus the same defects in files the workstream did not own.

Stacked on #147 (itself on #146) to keep each diff readable — this one contains only the rustmotion-components changes. Rebase to chantier/audit-remediation as the stack lands.

The findings

Reveal ignored start_at. chart, heatmap, sparkline and treemap drove their animation off raw scene time, so a component with start_at: 2.0 was already fully drawn the moment it appeared. Now measured from start_at, matching Counter::ramp_progress.

progress ignored its layout box. It painted at the declared width/height rather than what taffy computed, so a bar inside a sized container overflowed or under-filled — measured 299px of ink in an 800px box, and a ring centred at (149.5, 9.5) in a 60×60 box.

stacked_bar had no signed extent. Negative totals rendered outside the box: ink at y=[292..599] in a 300px-tall box. Stacks now grow either side of an anchored zero, like bar already did.

heatmap renormalised min→max. A uniform grid of 5.0 painted identically to a grid of 0.0, and color_scale did not mean the absolute 0..1 scale the docs describe. Renormalisation removed.

A flat sparkline glued itself to the bottom edge. Dividing by a floored range normalises every point to 0, which reads as "collapsed to zero" rather than "unchanged". Flat series now centre.

Axis labels drifted. Collected with filter_map, so one datum without a label shifted every subsequent label onto the wrong bar — AAA centred at 104 above a bar centred at 58.5.

treemap overprinted. Label and value shared one baseline. Fragments now stack.

Same bug, other files — fixed here rather than deferred

The workstream owned seven files; three of these defects live in files it did not. Fixing only the owned copies would have left an identical bug behind a green test suite, so they were applied at integration:

File	Defect
gauge.rs, dot_map.rs	progress_at ignoring start_at
chart/line.rs (×2), chart/waterfall.rs	filter_map label drift
stat.rs	flat-series divisor, in its inlined sparkline

Two things found beyond the brief

interpolate_color resolved t = 1.0 to the second-to-last colour: the local fraction was recomputed from the clamped segment index. Same function, same component, and the adversarial verifier had already produced the evidence — fixed alongside.

A defensive .max(0.0) on the circular-progress radius: it became reachable only once the ring started honouring its real layout box, which can now be arbitrarily small where the declared defaults never were.

Structural note, not addressed here

progress_at now exists in seven near-identical copies (Counter, Chart, Heatmap, Sparkline, Treemap, Gauge, DotMap), and the flat-series scale in three. The rule of three is well past — a crate-level ramp_progress(start_at, duration, time) would stop the next component reintroducing the same defect. Deliberately not done in this PR: it is a refactor across eight files and would bury sixteen bug fixes under it. Filed as follow-up work on #142.

Verification

cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace — 752 passed, 0 failed, re-run by the orchestrator on the integrated tree. Every fix written red first; the failure output for each is recorded in the workstream report.

[LeadcodeDev]

Recreated against chantier/audit-remediation directly: merging #146 with --delete-branch removed the base branch of the stack and closed the dependent PRs. Superseded — see the replacement PR.