fix(build): project-relative board_build.partitions CSV not found (resolved against framework tools dir)

[zackees]

Context

NightDriverStrip sets board_build.partitions = config/partitions_custom_noota.csv (project-relative, standard PlatformIO semantics). Every build in the #942 baseline logs:

WARN fbuild_build::esp32::orchestrator::boot_artifacts: no partitions source: csv=/root/.fbuild/prod/cache/platforms/framework-arduinoespressif32/.../esp32-core-3.3.9/tools/partitions/config/partitions_custom_noota.csv gen_tool=.../gen_esp32part.py

i.e. the CSV path is resolved only against the framework's tools/partitions/ directory. Consequences:

1. Wrong partition table: the custom table is silently ignored (build falls back), so flashed layouts won't match the project's config — user-facing correctness bug.
2. Perf side-effect: partitions.bin is missing from the final artifact set, so the ESP32 fast-path fingerprint is never persisted ("skipping ESP32 fast-path persistence because final artifacts are incomplete") and the whole-build short-circuit (perf(build): no-change rebuild recompiles the entire sketch + core variant (~103s; should be seconds) #951) can never engage for such projects.

Proposal

Resolve board_build.partitions first against the project dir (PlatformIO behavior), then fall back to the framework tools/partitions/ directory for named built-ins (default.csv, huge_app.csv, ...). Emit a hard error (not a WARN) when an explicitly configured CSV is not found anywhere.

Acceptance criteria

• NightDriverStrip demo builds generate partitions.bin from the project's config/partitions_custom_noota.csv.
• Built-in names (default.csv etc.) still resolve from the framework dir.
• Missing explicit CSV fails the build with a clear error.
• Unit test for project-relative resolution.
• The "skipping fast-path persistence" warning disappears in the meta: profiling-driven build performance burndown (Docker Linux, NightDriverStrip, cold vs hot cache) #942 harness for NightDriverStrip.

Decisions

• Severity: HIGH — silently flashing the wrong partition table is a correctness bug beyond the perf impact.

Part of #942 (blocks the #951 fast-path fingerprint persistence for projects with custom partitions).