feat(health): stop alarming for a fall the code did not cause

[RaghavChamadiya]

About half of a file's code-health deduction comes from git-derived markers —
churn, co-change, ownership, prior fixes — which rise as a file is worked on. So
a week of refactoring dropped the headline and the trend printed "Declining
health." in error red at the person who did the work. That is the reading this
removes.

A decline that names its cause

A fall now comes back as a third kind, history_drag, when the composite fell,
history is the driver, and history is the only half that moved down. It keeps
every number and swaps the reading:

Code health dropped 0.60 points vs. snapshot 5 ago (7.60 → 7.00), and code
shape improved 0.30 over the same window. Change history is the only half that
moved down, and no edit to these files settles it.

It takes the neutral treatment history findings already have in the file drawer
rather than the error colour, in the web view and the CLI alike.

Both halves are checked, not the driver alone. driver names the half that
moved furthest, which is not the half that moved down: the halves are deduction
means and the score is a mean of clamped scores, so on a repo with floored files
they can disagree about direction. Muting the alarm on that basis would have
blamed a fall on a half that had just improved.

Maintainability is watched

It is the one figure that is pure code shape, nothing alerted on it, and its
fall is the one that always deserves the alarm. It has no halves to split, so it
never softens. Both detectors now run over the three metrics uniformly, skipping
any a snapshot never recorded rather than reading a missing figure as zero, and
the composite is named "Code health" in their messages instead of "Average
Health".

The alert lead no longer names a figure. Three different things raise this alert
and the message says which, so a fixed "Declining health." above "Maintainability
dropped 0.60 points" contradicted itself.

The trend leads with the number a refactor moves

Maintainability is the heaviest line; drawing the composite loudest emphasised
the number that misleads. That left the history band looking as though it bounded
maintainability, so the band's edges are stroked in the code-health colour it
actually belongs to — which also states what the band means, since its upper edge
is where the score would sit with no history against it.

Verification

• 54 Python trend and projection tests, 21 UI tests, root npm run type-check
  and ruff check all green after rebasing onto the current headline work,
  which this overlapped on the trend view.
• Alert rows read out of the rendered page in light and dark, with all three
  kinds side by side.