snap: migrate builds to core26

[kvnloo]

Fixes #7199

Description of the change

Migrates the Snap base from core24 to core26. The existing Python build dependencies now run in a project-local virtual environment so the build does not invoke externally managed system pip under PEP 668.

This replacement removes the unnecessary test file from #7238. Each AI-assisted commit includes the BOINC-required Assisted-by: Hermes Agent:gpt-5.6-sol trailer.

Verification completed

• Reproduced system pip install --user failing with externally-managed-environment in Ubuntu 26.04
• Reproduced the project-local venv path succeeding for pip and Jinja2
• BOINC source-code check passed
• BOINC trailing-whitespace check passed
• YAML parsing and semantic configuration checks passed
• git diff --check upstream/master...HEAD passed
• Independent policy and scope review passed

Draft status

A complete Snap package build has not yet been verified at this exact head. This PR remains draft until that build path is green.

AI assistance

Hermes Agent using gpt-5.6-sol assisted with investigation, implementation, validation, and review. I reviewed the final one-file diff and the commands listed above.

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Summary by cubic

Migrates the Snap base from core24 to core26 and isolates Python build dependencies in a project-local virtual environment. Previously the build used pip --user and $HOME/.local/bin; now it creates .venv under $CRAFT_PART_BUILD and installs pip and jinja2 there to avoid PEP 668 externally-managed-environment failures on Ubuntu 26.04.

• Adds python3 and python3-pip to build-packages; keeps python3-venv; updates PATH to use .venv/bin.
• Replaces pip --user installs with python3 -m venv and python -m pip install -U pip jinja2.
• Builders must support base: core26 in Snapcraft; verify a full snapcraft build before promotion.

^{Written for commit 1964c1a. Summary will update on new commits.}

[kvnloo]

Closing this replacement draft. The cleaner repository practice is to keep the existing review thread and update the contributor-owned PR branch with a lease-guarded history rewrite. I will restore #7238 with the corrected, policy-compliant commits.