Implement canonical changelog area hierarchy (/) with filename encoding (~)

[Copilot]

Changelog tooling documented hierarchical areas (dns/cares in config, dns~cares in filenames) but never implemented the mapping, causing valid nested areas to fail validation and emit incorrect area: values in generated YAML. This change wires the convention through parsing, checking, and docs.

• Shared area encoding contract (envoy.base.utils)

  • Added canonical/filename separators and helpers:
    • AREA_SEPARATOR = "/"
    • AREA_FILENAME_SEPARATOR = "~"
    • area_from_filename(), area_to_filename()
  • Updated AChangelog.data_from_entry_map() to decode filename areas before emitting entry data, so generated YAML uses canonical IDs (e.g. dns/cares).

• Checker behavior (envoy.code.check)

  • check_entry_filename() now validates filename-area syntax with encoded charset ([a-z0-9_\-~]+), decodes ~ -> /, then validates against configured areas.
  • Unknown-area errors now show decoded + filename form when they differ:
    • Invalid area 'dns/cares' (from filename 'dns~cares')

• Areas config/title validation

  • Kept canonical area-key rule unchanged ([a-z0-9_\-/]+; ~ remains invalid in keys).
  • Added title rules:
    • title must not contain ~
    • if area key contains /, explicit non-empty title is required

• Documentation

  • Added a concise “Changelog entry areas” section to py/envoy.code.check/README.rst describing canonical IDs, filename encoding, and title requirements.

# canonical config key
area = "dns/cares"

# filename form
filename_area = area_to_filename(area)   # "dns~cares"

# checker/data path canonicalization
canonical = area_from_filename(filename_area)  # "dns/cares"

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