Add a daily movement score and heart-rate training load

[ak710]

Closes #116

Merge order: 6 of 9. Branches off main, independent of everything before it.

Movement score

Four contributors scored against the user's own goals, not a population target: steps (35), active minutes (30), active energy (20), movement spread through the day (15). The goals already exist and are editable; a fixed 10,000 steps would tell a marathoner and someone recovering from surgery the same thing.

Exceeding a goal is never penalised — overreaching is what training load is for.

The regularity contributor separates a day that hit its step goal in one gym session from one that moved throughout: 250 steps/hour (the stand-hour convention) over 08:00–22:00, judged against the hours the ring actually reported buckets for, so a ring taken off at lunch isn't scored for the afternoon it never saw.

Missing contributors leave the denominator: a ring-history day has no trustworthy calorie figure (buildTodaySummary already blanks it) so it's scored out of 80; a ring with no intraday buckets, out of 85.

Training load

Edwards' summated heart-rate zones, not Banister's TRIMP. Banister needs a reliable average HR over a bounded session; all-day ring data is sparse, irregularly spaced and has no session boundaries, so Edwards — which only needs time in each zone — degrades far better. Zone floors match the ones the workout summary already draws.

Each reading is credited the gap to the next, capped at ~2× the median spacing and never over five minutes. Without that, two all-day readings 12 h apart become "12 hours of zone-1 work" — testAnOvernightGapIsCappedNotCredited.

Acute (7-day) vs chronic (28-day) mean daily load, with the usual bands.

Days with no readings are excluded from both means, never counted as rest. A week the ring wasn't worn is not a week of recovery, and averaging in zeros would manufacture a "detraining" reading out of a charging cable — testUnwornDaysAreExcludedNotCountedAsRest. The ratio is withheld entirely below 14 of 28 covered days.

Placement

Activity tab, under the rings they summarise — not Today, which is already a dense tile grid. Breakdown disclosed on tap. Absent entirely on a day with no activity row, since an empty dial reads as a zero.

Testing

ActivityScoreTests (14) + TrainingLoadTests (13). Full suite green (898). Docs: docs/project/activity-score.md.

Two SwiftLint statement_position violations my if/else chains introduced are fixed as switches — and converting the band chain changed the boundary semantics, so testBandBoundariesAreInclusiveAtTheTop now pins that 1.3 stays "steady".