feat(ipip-0499): add test fixtures section with deterministic CIDs

- add verification process for testing profile compliance
- add test vectors for unixfs-v0-2015 profile (5 CIDs)
- add test vectors for unixfs-v1-2025 profile (5 CIDs)
- document HAMT threshold behavior (> not >=)
- document balanced vs trickle DAG layout with ASCII diagrams
- add trickle layout CID for reader compatibility testing
- reference existing test vectors from UnixFS spec (empty dir, symlink)

Author: lidel

src/ipips/ipip-0499.md

@@ -1,6 +1,6 @@
 ---
 title: 'IPIP-0499: UnixFS CID Profiles'
-date: 2026-01-14
+date: 2026-01-23
 ipip: proposal
 editors:
   - name: Michelle Lee
@@ -232,12 +232,72 @@ As an alternative to profiles, users can store and transfer CAR files of UnixFS
 
 ## Test fixtures
 
-TODO
+Test fixtures allow implementations to verify profile compliance by comparing CIDs produced for identical input data.
 
-List relevant CIDs. Describe how implementations can use them to determine
-specification compliance. This section can be skipped if IPIP does not deal
-with the way IPFS handles content-addressed data, or the modified specification
-file already includes this information.
+### Verification process
+
+1. Fetch the reference DAG by CID from the IPFS network or a gateway
+2. Export the DAG content to the local filesystem (e.g., `ipfs get <CID>`)
+3. Re-import using the implementation under test with the specified profile
+4. Confirm the resulting CID matches the reference
+
+### `unixfs-v0-2015` profile
+
+| CID | Description |
+|-----|-------------|
+| `Qmf412jQZiuVUtdgnB36FXFX7xg5V6KEbSJ4dpQuhkLyfD` | Small file: `hello world` string, dag-pb wrapped leaf |
+| `QmUbBALi174SnogsUzLpYbD4xPiBSFANF4iztWCsHbMKh2` | File at max links: 44544 KiB file, 174 × 256 KiB chunks (single-level DAG) |
+| `QmepeWtdmS1hHXx1oZXsPUv6bMrfRRKfZcoPPU4eEfjnbf` | File over max links: 44800 KiB file triggering DAG rebalancing (multi-level DAG) |
+| `QmX5GtRk3TSSEHtdrykgqm4eqMEn3n2XhfkFAis5fjyZmN` | Directory at HAMT threshold: links-bytes size = 262144 (basic directory) |
+| `QmeMiJzmhpJAUgynAcxTQYek5PPKgdv3qEvFsdV3XpVnvP` | Directory over HAMT threshold: links-bytes size = 262145 (HAMT sharded) |
+
+### `unixfs-v1-2025` profile
+
+| CID | Description |
+|-----|-------------|
+| `bafkreifzjut3te2nhyekklss27nh3k72ysco7y32koao5eei66wof36n5e` | Small file: `hello world` string, raw leaf |
+| `bafybeihmf37wcuvtx4hpu7he5zl5qaf2ineo2lqlfrapokkm5zzw7zyhvm` | File at max links: 1024 MiB file, 1024 × 1 MiB chunks (single-level DAG) |
+| `bafybeihmzokxxjqwxjcryerhp5ezpcog2wcawfryb2xm64xiakgm4a5jue` | File over max links: 1025 MiB file triggering DAG rebalancing (multi-level DAG) |
+| `bafybeic3h7rwruealwxkacabdy45jivq2crwz6bufb5ljwupn36gicplx4` | Directory at HAMT threshold: block-bytes size = 262144 (basic directory) |
+| `bafybeiegvuterwurhdtkikfhbxcldohmxp566vpjdofhzmnhv6o4freidu` | Directory over HAMT threshold: block-bytes size = 262145 (HAMT sharded) |
+
+### HAMT threshold behavior
+
+The HAMT threshold comparison uses `>` (strictly greater than), not `>=`. A directory with estimated size exactly equal to the threshold (262144 bytes) remains a basic directory. Only when the size exceeds the threshold does it convert to HAMT.
+
+### DAG layout
+
+Both profiles use the `balanced` layout where distance from root to each leaf is the same number of hops.
+
+```
+balanced:                              trickle:
+
+         root                                root
+        /    \                         /   /  |  \   \
+     node     node                  leaf node leaf node ...
+    / | \     / | \                      |         |
+ leaf leaf leaf leaf ...              node       node
+                                      / \        / \
+                                   leaf leaf  leaf leaf
+ ^^^^^^^^^^^^^^^^^^^^^^
+    uniform depth                    ^^^^     ^^^^^^
+                                     varying leaf depth
+```
+
+Implementations MAY use below CIDs to verify DAG layout handling. The `trickle` layout is not part of official profiles, but is included for verifying that UnixFS readers can parse non-standard DAG layouts produced by other software in the ecosystem:
+
+| CID | Description |
+|-----|-------------|
+| `QmepeWtdmS1hHXx1oZXsPUv6bMrfRRKfZcoPPU4eEfjnbf` | Balanced (v0): 44800 KiB file, 175 × 256 KiB chunks, depth 2 |
+| `bafybeihmzokxxjqwxjcryerhp5ezpcog2wcawfryb2xm64xiakgm4a5jue` | Balanced (v1): 1025 MiB file, 1025 × 1 MiB chunks, depth 2 |
+| `QmbqR6g7YCpndLcCZZXkGyj13uMcSrwDYUvZ47vJqBVjUH` | Trickle (v0): 45 MiB file, 180 leaves at varying depths (1-2) |
+
+### Additional test vectors
+
+For the following cases, see existing test vectors in the [UnixFS spec](https://specs.ipfs.tech/unixfs/#appendix-test-vectors):
+
+- **Empty directory**: well-known CIDs `QmUNLLsPACCz1vLxQVkXqqLX5R1X345qqfHbsf67hvA3Nn` (v0) and `bafybeiczsscdsbs7ffqz55asqdf3smv6klcw3gofszvwlyarci47bgf354` (v1)
+- **Preserved symlink**: [`QmWvY6FaqFMS89YAQ9NAPjVP4WZKA1qbHbicc9HeSKQTgt`](https://github.com/ipfs/gateway-conformance/raw/refs/tags/v0.8.1/fixtures/path_gateway_unixfs/symlink.car) (v0)
 
 [kubo]: https://github.com/ipfs/kubo
 [boxo]: https://github.com/ipfs/boxo