feat: emit PEP 691 JSON simple repository pages

[aiolibsbot]

What

Write index.v1_json alongside each index.html so the generated tree also
serves the PEP 691 JSON Simple Repository API
(application/vnd.pypi.simple.v1+json).

Why

Pip 22.0+ negotiates for JSON via the Accept header — it's smaller, faster
to parse, and now the de-facto API for modern installers. The HTML pages
remain the source of truth for older clients.

How

• New json_generator.py builds the root index and per-project payloads using
  the same canonicalisation/natsort ordering as the HTML generator. Optional
  fields (requires-python, core-metadata, size) are emitted only when
  set, and core-metadata: true is preserved.
• IndexMaker.generate_index_pages writes both files atomically via the
  existing write_utf8_file helper.
• README documents the nginx Accept-header rewrite needed for static hosting.

Testing

• New unit tests in tests/test_json_generator.py cover canonicalisation,
  optional-field omission, and the core-metadata: true passthrough.
• New end-to-end assertion in tests/test_index.py verifies the JSON files
  exist with correct mode and content for both root and project pages.
• Full suite: 20 passed.

🤖 Generated with Claude Code

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Quality Report

Changes: 5 files changed, 212 insertions(+), 6 deletions(-)

Code scan: clean

Tests: failed (command not found)

Branch hygiene: 1 issue(s)

• Branch is not pushed to remote

Generated by Kōan post-mission quality pipeline