fix(depot): add ltx decode diagnostics

[NathanFlurry]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[NathanFlurry]

• fix(pegboard): align drain grace defaults with stop threshold #4958 : 2 dependent PRs (#4960 , #4966 )
• fix(rivetkit): preserve bridge error messages #4959
• fix(rivetkit): fire abort signal on shutdown finalize #4957
• fix(rivetkit): use startup kv preload #4956
• fix(client): flatten skipReadyWait option #4955
• ci(publish): add manual preview publish flow #4954
• fix(depot): add ltx decode diagnostics #4953 👈 (View in Graphite)
• fix(universaldb): preserve plain set values #4952
• fix(universaldb): align versionstamp operand encoding #4951
• fix(rivetkit): export database provider types #4950
• wip: kitchen sink agentic loop finish #4949
• chore(rivetkit): note duplicate admission logic #4948
• test(rivetkit): cover bypass fetch header forwarding #4947
• feat(kitchen-sink): add actor probe controls #4946
• fix(gateway): split actor response wait errors #4945
• fix(guard): handle bypass preflight before query parse #4944
• feat(kitchen-sink): add mock agent event log copy #4943
• fix(kitchen-sink): default mock agent to local endpoint #4942
• test(rivetkit): enable driver lifecycle regressions #4941
• test(rivetkit): verify cached websocket db refs exceed grace #4940
• fix(rivetkit-core): keep sleeping actors reachable during grace #4939
• test(rivetkit): cover async websocket close db access #4938
• fix(rivetkit): keep actors awake until keepAwake work finishes #4937
• fix(rivetkit): persist native onSleep state on errors #4936
• fix(rivetkit-core): allow dispatch during sleep grace #4935
• fix(rivetkit): make duplicate sleep requests idempotent #4934
• main

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[claude]

Code Review: fix(depot): add ltx decode diagnostics

Overview

This PR improves error diagnostics when LTX blob decoding fails by including the source key (hot path only), blob length, and hex dumps of the first/last 64 bytes in the error context. It is a small, focused diagnostic improvement with no behavior changes.

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Code Quality

• Adding context inside .with_context(|| ...) closures is correct. The closure is lazy and only evaluated on failure, so the hex formatting cost is zero on the happy path.
• Using saturating_sub for tail_start correctly avoids underflow on short blobs.
• The crate::compaction::shared::hex_lower full-path references work, but per the project convention, imports should be added at the top of the file rather than referenced inline inside functions.

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Potential Issues

Head/tail overlap on short blobs: When len <= 64, both head_n == len and tail_start == 0, so the head and tail hex strings will be identical: the entire blob printed twice. This is redundant noise in the error message. Consider collapsing to a single blob= field when the blob is short enough to fit in one window, or only emitting the tail when tail_start > head_n.

Asymmetric diagnostics: The hot-path (read.rs) includes source_key in the error context explicitly. The cold-path (read/cold.rs) already embeds object_key directly in the format string prefix. This appears intentional rather than an oversight.

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Minor Style

• The PR description checklist is entirely unchecked. A brief note on what scenario triggered this change would be useful context for reviewers and future history.

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Assessment

The change is correct and valuable for debugging LTX decode failures in production. The only substantive concern is the duplicated head/tail hex output for blobs shorter than 64 bytes. Everything else is a minor style note. Good to merge once the overlap case is handled or explicitly accepted as-is.