Add an optional hybrid RW mode for LSMT writable layers.

[ZarrianShi]

What this PR does / why we need it:

This PR adds an optional hybrid RW mode for LSMT writable layers.

When enabled, writes reuse existing non-zero mappings in the current writable layer by overwriting their data payload in place. Ranges not covered by the writable layer—including holes, zeroed mappings, and mappings only present in lower layers—continue to append data and new index records.

This reduces RW layer growth for overwrite-heavy workloads while preserving the existing append-only behavior as the default. It does not introduce online compaction.

The change includes:

• A --hybrid option for creating a hybrid RW upper layer.
• A persisted writable-layer header flag for hybrid RW mode.
• Writable-layer-only range iteration, so lower-layer data is never overwritten.
• Hybrid range handling for partial overlap, holes, zeroed ranges, and stacked layers.
• Unit tests covering single/multiple upper ranges, internal holes, exact boundaries, zeroed mappings, and lower-layer isolation.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):

Fixes #442

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?