[Tyrie Vella (tyrielv)]

Problem and Context

Hydration status adds placeholder and modified-path counts, then compares those sums with index-based totals. These sources count different populations.
New paths, overlapping records, and retained folders after branch switches can exceed the totals without a calculation failure.
Current status handling rejects those results. The diagnostic contract from #2144 is already on master.

Background hydration also requests modified paths through the mount's own pipe. If the request arrives before mount readiness, the mount rejects it.
Its disk fallback conflicts with the mount's open writer. A clean status cache can then prevent another hydration refresh.

Changes

• Accept complete, nonnegative observations even when their sums exceed the totals. Reject incomplete summaries that the earlier count-only policy accepted.
• Collect both denominators and share acceptance and display across the summary, cache, CLI, and hook-linked code. Cap only the display at 100%. Identify excess counts and display n/a when a positive sum has a zero denominator.
• Change usable status output to Estimated hydration: files ...; folders .... Apply that format to gvfs health --status even when the status-hook feature is disabled.
• Preserve raw counts, denominator sources, signed differences, and positive excess in diagnostics. Use 64-bit sums without changing count populations, wire count fields, database records, or the circuit-breaker policy.
• Read the mount's existing in-memory modified paths during background hydration. Preserve the pipe and disk fallback for external health calculations.
• Keep cache lookups free of calculations and repository scans. Preserve the hook and cache lookup budgets.
• Update the functional test-output filter for the new status prefix.
• Add unit tests for count drift, unavailable sources, arithmetic boundaries, zero denominators, exact percentages, fractional truncation, and protocol compatibility. These tests assert shared CLI and hook behavior and raw cache diagnostics.
• Add unit tests for populated and empty in-memory modified paths, including empty-entry filtering. Extend the disabled-feature test to reject provider reads.
• Add functional tests that retain an obsolete placeholder folder with an untracked file after a branch switch. The tests distinguish that folder from a clean removed folder and verify cached status, forced fallback, and repeated cache reuse.
• The functional tests use distinct paths for cached and fallback cases. They wait for a stable cache identity and delete their test-owned paths. The tests report the last mount calculation on timeout.
• Document estimate semantics, source completeness, client compatibility, and the existing disk-reader limitations.