PR builds drop their superseded caches; ABI comment collapsed per distribution

[Tobias-Fischer]

Follow-up to #92 (pushed just after it was merged).

• Superseded build caches: each PR build saved a new cache entry per run and attempt, and the older entries of the same pull request/distribution/platform stayed — never restored again — until the repository's 10 GB limit evicted entries, current ones included, so full rebuilds lost their progress (the cache was at 11 GB, 3.5 GB of it superseded build caches; I deleted those by hand). After saving its cache, a build job now deletes the older ones (actions: write for the build job; skipped on forks' read-only token).
• ABI check comment: it ran to ~500 lines. Each distribution is now one <details> with its counts in the summary line (e.g. humble linux-64: 1 to review (changed headers), 128 can be bumped on their own), open only when dependents need a rebuild; the explanation is collapsed into "About this check".

Builds only rolling (build:rolling): nothing here changes a package.

🤖 Generated with Claude Code

[github-actions]

ABI check (linux-64)

rolling linux-64: **1 need their dependents rebuilt**

package	released → PR	verdict	details
rplidar-ros	2.1.0 (build 20) → 2.1.4	incompatible: removed or changed symbols: rebuild the dependents	removed: librplidar_composition_node.so rebuild 0 dependents (of 0 released dependents, these link to it): none 2 headers changed (check for layout/inline changes): -rplidar_ros/rplidar_node.hpp, -rplidar_ros/visibility.h pins changed: numpy >=1.23,<3 → >=1.25,<3

Compared with the newest builds on https://repo.prefix.dev/tobias-fischer/robostack-distro-test/linux-64 and https://repo.prefix.dev/robostack-rolling/linux-64.

About this check

Each package this pull request built, compared with its newest released build (tools/abi_check.py, libabigail):

• compatible / additions: the package can be bumped on its own;
• soname / incompatible: the packages that depend on it need a rebuild (build number bump) in the same pull request.

Exported symbols are compared (the released packages are stripped, so struct layouts aren't); unchanged installed headers rule out layout and inline changes, changed ones are listed for review. Dependents to rebuild are the released ones linking to a changed library. A changed pin is part of the comparison.