Fix TARBALL_INCLUDE config default drift (7r0, scoped down)

apply_config_defaults fell back to TARBALL_INCLUDE=no while the user-facing
src/shuriken.default.conf documents (and --init writes) TARBALL_INCLUDE=yes.
The two disagreed, so an album generated from a config that omitted the key
silently dropped the tarball despite the documented default saying otherwise.

'yes' is the authoritative default: the original definition was
`declare -r TARBALL_INCLUDE=yes` (tarball inclusion was on from the start);
the ':-no' fallback was introduced later during a refactor and drifted from
the documented intent. Align apply_config_defaults to ':-yes' and add a
comment explaining the invariant and the history.

Tests: add test_tarball_include_default_matches_init_config asserting the
effective default agrees between a fresh --init config and apply_config_defaults
(and is 'yes'); update the omitted-runtime-defaults --print-config expectation
to the corrected 'yes'. bin/shuriken regenerated via just build.

This is the scoped-down drift fix only. The full CONFIG_SPECS registry plus
cfg_get/SHURIKEN_CFG access-layer rewrite proposed in 7r0 remains DEFERRED.

just test, just shellcheck, just check-generated and git diff --check all pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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