ci: move test-build lane from Python 3.8 to 3.14

[lmeyerov]

Summary

First incremental PR under #1110's policy split:

• preserve PyGraphistry runtime compatibility policy at Python 3.8+
• keep broad compatibility testing lanes in CI
• move a non-compat operational lane (test-build) off EOL Python

Changes in this PR:

• test-build: setup-python 3.8 -> setup-python 3.14
• lockfile: build-py3.8.lock -> build-py3.14.lock
• CHANGELOG.md development note

Why

test-build is a packaging/operational lane (python -m build, wheel marker checks), not the compatibility lane of record.

We still retain explicit 3.8 compatibility coverage elsewhere in CI (python-lint-types, test-minimal-python, test-core-python, test-graphviz).

Validation

• Investigated prior CI perf references: ci: cache generate-lockfiles to skip ~47s on unchanged deps (#1076) #1083 and ci: speed up job matrix (issue #1082) #1084.
• Ran a concrete 3.14 trial in this PR branch with full CI green.
• test-build job timing comparison (observed):
  • prior 3.10 run: ~14s / 18s
  • corrected 3.14 run: ~19s / 20s

3.14 is slightly slower in this lane, but stable and green; delta is small.

Scope

• .github/workflows/ci.yml (test-build lane modernization)
• CHANGELOG.md

Related

• Refs CI/docs Python modernization with explicit 3.8 compatibility split #1110

[lmeyerov]

All checks are green and this PR is ready to merge.

Merge is currently blocked from this automation context due GitHub scope/rule restrictions on workflow-file updates.

Error:
refusing to allow an OAuth App to create or update workflow .github/workflows/ci.yml without workflow scope

A maintainer merge with appropriate credentials should succeed.