refactor: collapse opts merging to single authoritative path

[zuqini]

Remove the DEEP_MERGE strategy and drop opts from merge_specs entirely. Function-form opts could never be resolved at merge time (the plugin arg doesn't exist yet), so resolve_opts already had to run at load time — the merge-time DEEP_MERGE pass just produced an orphaned value that no runtime code read for its value, only for its nil-ness. This left two parallel opts pipelines and made merged_spec.opts subtly wrong whenever function-form and table-form were mixed.

Now:

• merge_specs skips opts. merged_spec.opts is always nil.
• resolve_all computes entry.has_opts once, a boolean summary used by the three existence checks in plugin_loader and startup.
• run_config always calls resolve_opts(sorted_specs, plugin) — the single-spec fast path is gone, which has the side benefit of always handing config() a fresh table instead of a live reference to the spec's underlying opts.
• merge_spec_array now always folds from an empty base, so the returned table is always freshly allocated and never aliases an input spec.

Tests updated to reflect the new contract:

• opts_test.lua: assert merged_spec.opts == nil and check entry.has_opts / sorted_specs[1].opts instead.
• merge_test.lua: the old "merge_specs deep merges opts" test now asserts merge_specs skips opts and that resolve_opts is the deep-merge path.
• is_single_spec_test.lua: same treatment.