Move schemeshard index-related code to index/ subdirectory

[vitalif]

Changelog category

• Not for changelog (changelog entry is not required)

Description for reviewers

[ydbot]

Run Extra Tests

Run additional tests for this PR. You can customize:

• Test Size: small, medium, large (default: all)
• Test Targets: any directory path (default: ydb/)
• Sanitizers: ASAN, MSAN, TSAN
• Coredumps: enable for debugging (default: off)
• Additional args: custom ya make arguments

[github-actions]

⚪ 2026-04-10 16:56:46 UTC Pre-commit check linux-x86_64-relwithdebinfo for 8f6d733 has started.
⚪ 2026-04-10 16:56:52 UTC Artifacts will be uploaded here
⚪ 2026-04-10 16:58:20 UTC ya make is running...
🟡 2026-04-10 19:19:11 UTC Some tests failed, follow the links below. Going to retry failed tests...

Details

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
49260	45709	0	3	3536	12

⚪ 2026-04-10 19:19:27 UTC ya make is running... (failed tests rerun, try 2)
🟢 2026-04-10 19:24:02 UTC Tests successful.

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
14 (only retried tests)	14	0	0	0	0

🟢 2026-04-10 19:24:05 UTC Build successful.
🟢 2026-04-10 19:24:30 UTC ydbd size 2.5 GiB changed* by +408 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 1999e20	merge: 8f6d733	diff	diff %
ydbd size	2 662 508 592 Bytes	2 662 509 000 Bytes	+408 Bytes	+0.000%
ydbd stripped size	568 315 864 Bytes	568 316 440 Bytes	+576 Bytes	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[ydbot]

Backport

To backport this PR, click the button next to the target branch and then click "Run workflow" in the Run Actions UI.

Branch	Run
stable-25-4, stable-25-4-1, stable-26-1, stable-26-1-1
stable-26-1, stable-26-1-1
stable-26-1