fix(code-index): shrink text builds to available memory headroom

[ScriptedAlchemy]

Live deadlock after #1777

The cgroup authority is now correctly 26 GiB, but text projection still retried BudgetExceeded ~1–2 times/sec:

• old serving graph / live daemon RSS: 18–22 GiB,
• preferred text builder budget: 26 GiB / 8 = 3.25 GiB,
• supported builder floor: 1.5 GiB,
• high-watermark reserve: 2.6 GiB.

At 21 GiB RSS, ~2.4 GiB is available below the watermark: enough for the supported builder, not the preferred ceiling. open_published_head_or_begin_build nevertheless reserved the preferred figure all-or-nothing, so the stale graph prevented the replacement build that would release it.

Checked existing open PRs first: none implement flexible resident admission; #1577 is codec/decode work.

Fix

• reserve_resident_memory_up_to(preferred, minimum) computes available growth from the same modeled + observed + watermark ledger the atomic reservation uses and admits min(preferred, available) when it remains above the established 1.5 GiB floor.
• Source batch width and SQLite builder budget derive from the admitted amount, so the smaller reservation is real, not accounting-only.
• Reader and clone-successor reservations call the exact wrapper (minimum == preferred) and are unchanged.
• If less than the floor remains, return AuthorityUnavailable naming minimum/available; if the atomic reserve loses a race, preserve its exact admission failure. Only real control deadlines remain BudgetExceeded.

Evidence

Concrete 26 GiB/21 GiB case returns the available 2.4 GiB; 22 GiB case stays unavailable below floor. Flexible-budget + existing watermark/reservation/subdivision tests 4/4; all text-artifact tests 20/20; rustfmt and IDE lints clean.

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