Allow major-version mismatch toggle + version-compat gate in pick_build_path

[bdraco]

The 7b offloader Settings UI shipped (#574) with every toggle except Allow major-version mismatch. The compile-side enforcement gate hasn't landed either: pick_build_path doesn't consume StoredPairing.esphome_version today, so a remote receiver running a different major ESPHome version than the offloader is silently treated as eligible.

What's already in place

• The receiver advertises its esphome.const.__version__ through the peer-link handshake's intent_response payload on every session-open (Advertise receiver esphome_version through the peer-link handshake (7a-2 prerequisite) #557).
• The offloader captures it into StoredPairing.esphome_version (validator-capped at PAIRING_VERSION_MAX_LEN=64).
• The field is surfaced on PairingSummary so the Settings UI can show it.

What's missing

1. A version-compat gate in pick_build_path that filters peers whose esphome_version differs from the offloader's esphome.const.__version__ at the major level.
2. A user-facing "Allow major-version mismatch" toggle on the offloader Settings UI (per-pairing or master — TBD) that widens the gate.
3. The compile-time check that catches the mismatch before the receiver wastes CPU on an incompatible YAML.

Why deferred

Silently filtering eligible peers without an operator-visible override would be the wrong default — a user wouldn't know why their freshly-paired receiver suddenly stopped getting jobs. The gate and the toggle land together.

Acceptance

• pick_build_path exposes a version-compat gate (default-on) that drops mismatched peers from the eligible set.
• --no-X / explicit-on toggle on OffloaderRemoteBuildSettings + per-pairing override widens the gate.
• Settings UI surfaces both (master + per-row).
• Tests pin the matrix: matched-major eligible, mismatched-major filtered by default, override re-enables.