feat: accept the full compose-go duration grammar for healthcheck interval

[lesnik512]

Rewrites healthcheck.interval_seconds to parse the full compose-go duration grammar the way docker compose config v5.1.2 does — closing the compound/hour healthcheck-duration over-reject — and closes two hard-rule false greens found along the way.

Design and rationale: planning/changes/2026-07-16.03-healthcheck-duration-grammar.md.

What changed

• interval_seconds now parses the complete compose-go duration grammar → whole seconds: all units incl. d (days) and w (weeks), compound (1h30m), fractional (1.5d), and sign (-1h). interval paces compose2pod's own wait_healthy loop and never reaches podman, so it can honor the full set. timeout/start_period (emitted verbatim to podman's Go-parser --health-*, no d/w) are unchanged — a legitimate refusal under rule two.
• False green Port the compose-to-podman-pod converter into compose2pod #1 (whitespace): dropped the old .strip(). Padded durations (interval: " 5s ") were accepted while Docker rejects all whitespace — a hard-rule violation the flat matrix never probed (interval is nested). Guarded by a both-reject conformance corpus doc using a genuine demonstrator (a value the old strip-parser accepted).
• False green Adopt planning-convention v1.1.2 and migrate planning docs #2 (trailing newline): the anchored regex used $, which in Python matches before a trailing \n, so interval: | block scalars ("5s\n") slipped through. Switched to \Z; guarded by a block-scalar corpus doc.
• Overflow-safe (math.isfinite before int()), so a giant duration raises cleanly.

Verification

• 1368 tests @ 100% line coverage; just lint-ci, just check-planning clean.
• Conformance: healthcheck_compound_duration/healthcheck_hour_duration flip over-reject → conformant (with dedicated both-accept assertions); the two new false-green guards are both-reject. Over-rejection count 6 → 4.

Follow-up (separate change)

The trailing-\n $-vs-\Z flaw is systemic across the other regex-anchored values.py grammars (_SIZE, _DURATION, _PORT, _STRICT_INT_STRING — mem_limit, ports, timeout, etc.), all pre-existing. Scoped out of this PR by design; a dedicated change will close the class.

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