Stabilize disk-persisted enum values

[Badrish Chandramouli (badrishc)]

Also writes-first RespCommand + freezes for format stability

RespCommand and several other enums have their numeric values persisted to disk (AOF, checkpoints, cluster config) and streamed to replicas. Because they were auto-numbered by declaration order, adding a member (even a read-only command) shifted every subsequent value, so data written by an older Garnet version could replay/recover as the wrong member on a newer version.

Changes:

• RespCommand: move write (mutating) commands into a dense, explicitly-numbered block (APPEND=1 .. BITOP_DIFF=120) immediately after NONE. Writes are the only persisted RespCommand values; reads/scripts/admin follow and may renumber freely. Boundary checks become positional ranges (IsReadOnly is now a branchless range check); RespCommandsInfo fast arrays re-anchor to FirstDataCommand.
• Object sub-op enums (Hash/List/Set/SortedSet Operation), GarnetObjectType, NodeRole and SlotState: freeze with explicit values + append-only guidance.
• GarnetObjectType: pin the custom-object-type base to a fixed 0x40 so adding a built-in type never shifts persisted custom-object ids; reserve 0xFC..0xFF for a future object-format version byte with a fail-fast read guard.
• RespInputHeader: give the object sub-operation id its own header byte (256/type instead of 5 bits) by dispatching COSCAN via type==All instead of header.cmd, freeing byte 1. Header stays 3 bytes; string/unified layout is unchanged.
• AOF: bump header version 3->4. Replay remaps legacy (v3) RespCommand values by name (LegacyRespCommand, fail-fast on rename) and relocates the v3 object sub-op id; a max-version guard rejects unknown-higher versions loudly.
• Add PersistedEnumStabilityTests (golden values, write-block invariant, sub-op capacity, custom-range, legacy mapping). Update the add-command skill and docs.

[Copilot]

Pull request overview

This PR stabilizes several enum numeric values that are persisted to disk (AOF/checkpoints/cluster config) or streamed to replicas, by explicitly freezing values and introducing compatibility handling for the AOF format. It also reorganizes RespCommand into a writes-first, explicitly-numbered persisted block to prevent unrelated additions from shifting persisted IDs.

Changes:

• Reordered RespCommand to a dense, explicitly-numbered write block (APPEND = 1 .. BITOP_DIFF = 120), updated read/write classification to use explicit ranges, and re-anchored fast command-info arrays to FirstDataCommand.
• Expanded object sub-operation capacity by moving RespInputHeader’s object sub-op id into a dedicated header byte, plus AOF v3→v4 replay remapping for legacy RespCommand numbering and legacy object sub-op packing.
• Froze additional persisted enums (object sub-ops, object types, cluster roles/states), and added golden stability tests + docs/skill guidance updates.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
website/docs/dev/range-index-resp-api.md	Updates developer docs to reflect writes-first RespCommand layout and range-based read/write classification.
test/standalone/Garnet.test/PersistedEnumStabilityTests.cs	Adds golden-value tests guarding persisted enum stability and legacy v3 mapping invariants.
libs/server/Storage/Functions/ObjectStore/ReadMethods.cs	Routes COSCAN via header.type == All to the stored object’s Operate for scan dispatch.
libs/server/Resp/RespCommandsInfo.cs	Re-anchors fast lookup arrays and offsets to FirstDataCommand.
libs/server/Resp/Parser/RespCommand.cs	Introduces explicitly-numbered write block, updates range constants, and makes IsReadOnly a two-sided range check.
libs/server/Resp/Objects/SharedObjectCommands.cs	Stops overriding cmd for COSCAN; relies on header.type == All signaling instead.
libs/server/Objects/Types/GarnetObjectType.cs	Freezes built-in object type values, documents reserved bands, and defines reserved object-format marker range.
libs/server/Objects/Types/GarnetObjectSerializer.cs	Adds fail-fast guard for reserved object-format marker bytes (0xFC..0xFF).
libs/server/Objects/SortedSet/SortedSetObject.cs	Freezes SortedSetOperation values and documents append-only persistence.
libs/server/Objects/Set/SetObject.cs	Freezes SetOperation values and documents append-only persistence.
libs/server/Objects/List/ListObject.cs	Freezes ListOperation values and documents append-only persistence.
libs/server/Objects/Hash/HashObject.cs	Freezes HashOperation values and documents append-only persistence.
libs/server/InputHeader.cs	Refactors RespInputHeader layout: dedicated subId byte + flags-only byte; updates accessors and docs.
libs/server/Custom/CustomObjectCommandWrapper.cs	Raises max custom-object subcommand id to 255 due to full-byte subId.
libs/server/Custom/CustomObjectBase.cs	Changes COSCAN detection to header.type == All instead of header.cmd.
libs/server/Custom/CustomCommandManager.cs	Pins custom object type id base to fixed 0x40 (reserved built-in band ends at 0x3F).
libs/server/AOF/LegacyRespCommand.cs	Adds name-based v3→current RespCommand mapping table for replay compatibility.
libs/server/AOF/AofProcessor.cs	Adds max-version guard and legacy replay translation for v3 RespCommand + legacy object sub-op packing.
libs/server/AOF/AofHeader.cs	Bumps AOF header version to 4 and defines max supported version.
libs/cluster/Server/Worker.cs	Freezes NodeRole numeric values and documents persistence requirements.
libs/cluster/Server/HashSlot.cs	Freezes SlotState numeric values and documents persistence requirements.
.github/skills/add-garnet-command/SKILL.md	Updates command-adding guidance for writes-first, explicitly-numbered persisted write block + persisted sub-op enums.