implements IPIP-499: add config options for controlling UnixFS DAG
determinism and introduces `unixfs-v1-2025` and `unixfs-v0-2015`
profiles for cross-implementation CID reproducibility.

changes:
- add Import.* fields: HAMTDirectorySizeEstimation, SymlinkMode,
  DAGLayout, IncludeEmptyDirectories, IncludeHidden
- add validation for all Import.* config values
- add unixfs-v1-2025 profile (recommended for new data)
- add unixfs-v0-2015 profile (alias: legacy-cid-v0)
- remove deprecated test-cid-v1 and test-cid-v1-wide profiles
- wire Import.HAMTSizeEstimationMode() to boxo globals
- update go.mod to use boxo with SizeEstimationMode support

ref: https://specs.ipfs.tech/ipips/ipip-0499/

add CLI flags for controlling file collection behavior during ipfs add:

- `--dereference-symlinks`: recursively resolve symlinks to their target
  content (replaces deprecated --dereference-args which only worked on
  CLI arguments). wired through go-ipfs-cmds to boxo's SerialFileOptions.
- `--empty-dirs` / `-E`: include empty directories (default: true)
- `--hidden` / `-H`: include hidden files (default: false)

these flags are CLI-only and not wired to Import.* config options because
go-ipfs-cmds library handles input file filtering before the directory
tree is passed to kubo. removed unused Import.UnixFSSymlinkMode config
option that was defined but never actually read by the CLI.

also:
- wire --trickle to Import.UnixFSDAGLayout config default
- update go-ipfs-cmds to v0.15.1-0.20260117043932-17687e216294
- add SYMLINK HANDLING section to ipfs add help text
- add CLI tests for all three flags

ref: ipfs/specs#499

add comprehensive test suite for UnixFS CID determinism per IPIP-499:
- verify exact HAMT threshold boundary for both estimation modes:
  - v0-2015 (links): sum(name_len + cid_len) == 262144
  - v1-2025 (block): serialized block size == 262144
- verify HAMT triggers at threshold + 1 byte for both profiles
- add all deterministic CIDs for cross-implementation testing

also wires SizeEstimationMode through CLI/API, allowing
Import.UnixFSHAMTSizeEstimation config to take effect.

bumps boxo to ipfs/boxo@6707376 which aligns HAMT threshold with
JS implementation (uses > instead of >=), fixing CID determinism
at the exact 256 KiB boundary.

- update boxo to ebdaf07c (nil filter fix, thread-safety docs)
- simplify changelog for IPIP-499 section
- shorten test names, move context to comments

switches to ipfs/boxo@6141039

changes since 5cf22196ad0b:
- refactor(unixfs): use arithmetic for exact block size calculation
- refactor(unixfs): unify size tracking and make SizeEstimationMode immutable
- feat(unixfs): optimize SizeEstimationBlock and add mode/mtime tests

also clarifies that directory sharding globals affect both `ipfs add` and MFS.

…s-2025

- fix typo in files write help text
- update boxo with CI fixes (gofumpt, race condition in test)

skip '@helia/mfs - should have the same CID after creating a file' test
until helia implements IPIP-499 (tracking: ipfs/helia#941)

the test fails because kubo now collapses single-block files to raw CIDs
while helia explicitly uses reduceSingleLeafToSelf: false

changes:
- run aegir directly instead of helia-interop binary (binary ignores --grep flags)
- cache node_modules keyed by @helia/interop version from npm registry
- skip npm install on cache hit (matches ipfs-webui caching pattern)

…s-2025

# Conflicts:
#	docs/examples/kubo-as-a-library/go.mod
#	docs/examples/kubo-as-a-library/go.sum
#	go.mod
#	go.sum
#	test/dependencies/go.mod
#	test/dependencies/go.sum

switches to boxo@main after merging ipfs/boxo#1088

switches to go-ipfs-cmds@master after merging ipfs/go-ipfs-cmds#315