[rupayon123]

After SetData(nil), reads see an empty configuration, but writing a top-level or literal dotted key panics on assignment to the nil root map. Nested writes already initialize their path.

Initialize the missing root map inside the existing locked write path. Readonly/invalid-key checks still reject before allocation, and hooks/cache behavior is unchanged.

Go 1.26.0 / macOS arm64 validation:

• Two failing write cases and three nested/readonly/invalid controls pass after the fix.
• Full coverage and race suites each pass 217 leaf tests with no skips.
• Twelve actual configuration/JSON scenarios complete three reset/reuse cycles each with no mismatches; pristine source fails eight scenarios.
• Vet, build, formatting, tidy and the YAML example pass. Staticcheck and Revive retain their unchanged baseline findings.

Module files are unchanged. Other supported Go/OS combinations remain unverified locally; this does not add broader external-map ownership or read-side concurrency guarantees.