Lift whole forest by absolute repo staleness

[thalida]

What

Tree height is normalized to a repo's own commit span, so a repo untouched for years still shows "young" trees at its newest end. This adds a scan-time staleness lift: a stale repo's whole forest reads old, while keeping the intra-repo oldest→newest spread.

staleness = clamp(daysIdle / STALE_HORIZON_DAYS, 0, STALENESS_CAP)   // daysIdle = scanned_at − newest commit
maturity  = staleness + (1 − staleness) · relT                        // relT = 1 − ageT (oldest = 1, newest = 0)
height    = MIN_HEIGHT + maturity · (MAX_HEIGHT − MIN_HEIGHT)

• asOf = manifest.scanned_at, not Date.now() — a live clock would drift tree height over time and break the deterministic goldens.
• Fresh repo is a no-op: staleness ≈ 0 → maturity == relT → byte-identical to today's repo-relative sizing.
• Stale repo: whole forest lifts toward MAX while preserving oldest→newest ordering. The oldest tree stays pinned at MAX (maturity 1); the newest floors at staleness.

Settings

The two tunables are Rebuild-routed TREES settings, in the Settings panel under Trees → Height by age:

Setting	Default	Range	Meaning
Staleness horizon (days)	730 (~2y)	30–3650	Idle time (newest commit → scan) for the forest to reach its oldest look
Staleness cap	0.85	0–1	Ceiling on the lift; < 1 so a dead repo never fully flattens. 0 turns the lift off

Design notes

• AgeRange carries the raw, settings-independent daysIdle; treeHeight derives the staleness lift from the current cfg horizon/cap. The tree renderer and the firefly orbit field both derive height from that one path (with the same TREES.value), so orbs never drift off their trees and either slider re-runs the rebuild with no extra wiring.
• The horizon is floored at 1 day so a mis-set 0 can't divide by zero.
• Threading: manifest.scanned_at → trees rebuild → createTreeRenderer, and → fireflies rebuild → placeFireflies. Both forward it to computeAgeRange; omitting it keeps daysIdle = 0.
• Golden bench: decorationGolden.bench.test.ts calls the renderer/placement without scanned_at, so daysIdle = 0 → digest byte-identical, no recapture needed. The bench now guards the fresh-repo no-op invariant.

Tests

TDD in treeEncoding.test.ts: daysIdle counting/clamp + ISO-datetime scanned_at; a fresh repo is unchanged; a stale repo lifts the whole forest while preserving ordering; the cap holds; the horizon/cap settings tune the lift (incl. cap 0 = off); a same-day (span 0) stale repo degrades sanely. Full suite green (2553), lint clean.

🤖 Generated with Claude Code