[docs]: broken links

[yaythomas]

Summary

zensical build --clean prints 19 validation warnings about broken
intra-doc links in docs/sdk-reference/. Every warning is one of
three mistakes:

• singular vs plural filename (steps.md, callbacks.md,
  child-contexts.md — the actual files are singular);
• a stale path from an earlier doc reorganization
  (../advanced/error-handling.md, error-handling.md,
  ../../testing/basic-tests.md, ../getting-started.md);
• one anchor slug that no longer matches its heading
  (#retrying-specific-errors vs ## Retry only specific errors,
  which Zensical slugifies to retry-only-specific-errors).

None of these break the published site (links render but 404 on
click), but they surface as build warnings and should resolve cleanly.

Broken links

#	File	Line	Broken link	Suggested fix
1	sdk-reference/error-handling/retries.md	111	#retrying-specific-errors	#retry-only-specific-errors
2	sdk-reference/observability/logging.md	406	../operations/child-contexts.md	../operations/child-context.md
3	sdk-reference/observability/logging.md	407	error-handling.md	../error-handling/errors.md
4	sdk-reference/operations/child-context.md	322	steps.md	step.md
5	sdk-reference/operations/map.md	689	steps.md	step.md
6	sdk-reference/operations/map.md	690	../advanced/error-handling.md	../error-handling/errors.md
7	sdk-reference/operations/parallel.md	736	steps.md	step.md
8	sdk-reference/operations/parallel.md	737	../advanced/error-handling.md	../error-handling/errors.md
9	sdk-reference/operations/step.md	431	../../testing/basic-tests.md	../../testing/index.md
10	sdk-reference/operations/wait-for-condition.md	7	steps.md	step.md
11	sdk-reference/operations/wait-for-condition.md	19	callbacks.md	callback.md
12	sdk-reference/operations/wait-for-condition.md	184	callbacks.md	callback.md
13	sdk-reference/operations/wait-for-condition.md	185	steps.md	step.md
14	sdk-reference/operations/wait-for-condition.md	186	../getting-started.md	../../getting-started/index.md
15	sdk-reference/operations/wait.md	24	steps.md	step.md
16	sdk-reference/operations/wait.md	35	callbacks.md	callback.md
17	sdk-reference/operations/wait.md	263	steps.md	step.md
18	sdk-reference/operations/wait.md	265	callbacks.md	callback.md
19	sdk-reference/operations/wait.md	266	../getting-started.md	../../getting-started/index.md

Repro

pip install zensical==0.0.40
zensical build --clean 2>&1 | grep -E 'Warning:|issues found'
# 19 issues found

Acceptance criteria

• zensical build --clean prints No issues found.
• Every listed link resolves to an existing page in the built site.