fix: compact Bind storage and terminal portal release

[bill-ph]

Summary

• retain each Bind frame once with compact parameter offsets
• interpolate text Bind values directly into Flight SQL without per-value Go strings or interfaces
• release terminal portal payloads and prepared-statement references while retaining cached RowDescription for Describe(P)

Follow-ups

• binary Bind allocation optimization
• per-connection guardrails and observability
• process-wide retained/in-flight memory admission
• transaction-aware Sync/transaction-end cleanup
• safe mutation result-format validation
• Execute(maxRows) fail-closed behavior and eventual PortalSuspended support

Validation

• go test ./server ./server/flightclient ./server/sqlcore
• go test -race ./server -run 'TestPortalLifecycle'
• go test ./server -run '^$' -bench '^(BenchmarkHandleBind27000Params|BenchmarkBindToFlightInterpolation27000Params)$' -benchmem
• just lint
• git diff --check

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	5	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +15 / -0
• Assertions: +92 / -3
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.