ci: collect EVE coverage per matrix job in Eden CI

[eriknordmark]

Description

Per-job coverage collection for the Eden test matrix plus a fan-in
merge job, intended to be consumed by a coverage-instrumented EVE
build on the lf-edge/eve side.

Each suite in the Eden test matrix expands into ~12 independent
runners with their own ./eden setup -> start -> stop -> make clean
lifecycle, so per-job collection is the only point at which to
capture covdata. After make clean the runner is wiped — there is no
later opportunity.

Two commits:

collect-coverage: preserve raw covdata in output

CollectEveCoverage SCP'd the raw covmeta.* / covcounters.*
files into a temp dir, ran go tool covdata textfmt on them, and
let the deferred cleanup wipe the binary files on return. Only the
text profile at <output-dir>/eden_e2e_coverage.txt survived.

Callers that want to merge coverage across multiple runs (e.g.
across the Eden test matrix) need the raw covdata so they can use
go tool covdata merge; the text profile alone only supports
concatenation, which is fine for a per-line summary but doesn't
aggregate the way merge does.

Copy the SCP target into <output-dir>/covdata/ instead of an
auto-deleted temp dir. The text-profile output path is unchanged.

ci: collect eden e2e coverage per matrix job

• run-eden-test composite action gains a Collect/Upload coverage
  pair, gated as best-effort (if: always() and || true), that
  invokes ./eden eve collect-coverage and uploads the resulting
  eden_coverage/ directory (text profile + raw covdata subtree) as
  artifact covdata-<suite>-fs-<fs>-tpm-<tpm>-attempt-<n>.
• test.yml gains a coverage-merge job that downloads every
  covdata-* artifact, runs go tool covdata merge on the raw
  covmeta.* / covcounters.* files to produce a single merged covdata
  directory, converts it to a text profile via covdata textfmt,
  and runs go tool cover -func for a per-package summary.

Companion PR

This change is the prerequisite for
lf-edge/eve#5950,
which adds an opt-in COVER=y EVE build (gated on a coverage PR
label) and wires eden-trusted.yml to consume the resulting
artifact. The EVE PR calls
lf-edge/eden/.github/workflows/test.yml@master, so this PR has to
land first; otherwise the tests-master-cover job on the EVE side
will run tests but emit no covdata-merged artifact.

How to test and validate this PR

Without a coverage-instrumented EVE artifact, the new steps run as
no-ops:

1. Confirm eden.yml (the eden-only CI path that consumes
   lfedge/eve:16.6.0) still passes after this change. The
   Collect coverage step warns ("no covcounters appeared")
   because zedbox isn't built with -cover, and the
   Upload coverage artifact step skips because the
   eden_e2e_coverage.txt hashFiles check is false. No new artifact
   is uploaded.
2. Confirm the coverage-merge fan-in job runs to completion and
   emits the "No raw covdata found across jobs" warning instead of
   failing.
3. Locally: build EVE with COVER=y and run a single suite via
   make eden-cover (or by passing
   --coverage-dir <dir> to eden test); confirm
   <dir>/eden_e2e_coverage.txt AND <dir>/covdata/coverage/
   (containing covmeta.* and covcounters.*) both exist after
   the run.
4. Once the companion EVE PR lands on master, exercise the full
   path end-to-end by labelling a coverage-instrumented EVE PR with
   coverage and confirming the covdata-merged artifact is
   produced at the end of the workflow run.

Notes

No user-facing changes. CI / dev-tooling only.