chore(contrib): update PKGBUILD version and include in bump_version.py

[amir-rezaei]

Description

This PR resolves issue #1159 by updating contrib/aur/PKGBUILD to the current repository version (0.26.0) and including it in scripts/bump_version.py.

Details

• Previously, contrib/aur/PKGBUILD remained hardcoded to 0.22.0 with a stale sha256 checksum, causing local AUR builds (makepkg) from the repository to fail.
• Updates pkgver in PKGBUILD to 0.26.0 and sets sha256sums to ('SKIP').
• Updates scripts/bump_version.py to rewrite pkgver in PKGBUILD, verify it during release checks, and stage it when creating release tags.

[Kohei-Wada]

Confirmed the staleness (pkgver=0.22.0 vs repo 0.26.0), and sha256sums=('SKIP') is fine here — #1159 lists it as an accepted option, and release.yml patches the file at publish time anyway.

Before merging, please add tests for the new bump_version.py behaviour, since scripts/ changes are exactly where a silent regression breaks a release:

1. update_pkgbuild_file rewrites pkgver and reports the change; --dry-run leaves the file untouched.
2. It is a no-op when pkgver already matches, and when the PKGBUILD is absent.
3. verify_no_old_versions flags a PKGBUILD still carrying the old version.

Verified locally against current main: lint, typecheck and the full suite pass (ruff is 0.16 on main now, so please rebase).