docs: add UPGRADING.md for moving from dryoc 1.x to 2.0

[brndnmtthws]

Adds UPGRADING.md, a single page for the 2.0.0 release notes to link to. It lists every breaking change between v1.0.0 and main, with a 1.x → 2.0 mapping for each. README.md gets one line linking to it, below the Quick Start Cargo.toml snippet. src/lib.rs is unchanged.

What the page covers

• Cargo features: serde is now a default feature; wincode → wincode_0_6 (wincode 0.5 → 0.6); u64_backend removed; the std/alloc split, which changes what default-features = false means; nightly no longer enables protected, and it needs nightly-2026-09-24 or later.
• Removed gen*, *_with_defaults, zero-key KeyPair::new/Default and StackByteArray::new, with the replacement for each.
• Fixed-size inputs: ByteArray is no longer implemented for slices or Vec, so convert with try_from.
• Imports and paths: algorithm modules no longer re-export types::*; keypair::protected, precalc::protected and protected::ptypes are private; alias renames (dryocbox::KeyPair, kx::KeyPair, sha3::Sha3*Digest, IncrementalSigner, sodium_increment).
• Box, signing, MAC, secret stream (Tag enum), key exchange, HKDF, hashing and password hashing changes; generic parameter order for callers that use a turbofish; sealed traits.
• Classic API: argument order in the three detached-open functions.
• A section on the changes that still compile: a detached-open call with a 16-byte ciphertext in the old order, the wider generichash length ranges, protected-memory deserialization now returning errors, and new #[must_use] warnings.

It leaves out renames of APIs added after 1.0.0 (kem, dryocsealedbox, XOF), since no 1.x user can have code using them.

Sources

• The BREAKING CHANGE footers and migration tables of refactor!: shared helpers, deduplicated impls, and removal of deprecated gen aliases (2.0.0) #153, chore!: remove unused u64_backend feature #172, fix!: remove all-zero keypair constructors and Default impls #174, feat!: rename wincode feature to wincode_0_6 #175, fix!: restrict ByteArray to fixed-size storage #176, feat!: no_std support with std and alloc features #178, feat!: enable serde by default #189, refactor(api)!: rename box open verbs and drop redundant box constructors #201, refactor(api)!: match libsodium argument order in classic detached open #203 and refactor(api)!: 2.0 API cleanup (signatures, stream Tag/Ed25519ph, aliases, must_use, generic order, re-exports and sealing) #204.
• cargo semver-checks --baseline-rev v1.0.0 --release-type minor (17 failing lints; I checked every removed or renamed item it reported against the page).
• A diff of the public type aliases and Classic pub fn signatures between v1.0.0 and main.

Verification

• Wire formats, checked against the published dryoc 1.0.0 from crates.io. A throwaway crate depended on both dryoc =1.0.0 (features serde, wincode) and this branch (wincode_0_6).
  • Same bytes from both versions for to_vec, serde_json and bincode (1.3): secretbox, box, key pair, signing key pair and signed message.
  • Same bytes from wincode 0.5 (1.x) and wincode 0.6 (2.0) for the secretbox and box.
  • Written by 1.x, read and opened by 2.0: a sealed DryocBox (from bytes and from serde), an XChaCha20-Poly1305 VecEnvelope (from bytes, serde and wincode), a secret stream with MESSAGE and FINAL tags, and a password hash (encoded string and serde).
  • The page's claim that wire formats are unchanged rests on these checks.
• 2.0 column. A second throwaway crate compiles and runs the 2.0 side of every table on this branch: the placeholder and try_from snippets, StackKeyPair/StackSigningKeyPair/StackKdf/HkdfSha256/HkdfSha512::generate, VecBox::from_parts/into_parts/open/open_to_vec, seal_to_vecbox, sign_to_vecbox, Ed25519phSigner, the borrowed-key MACs, the turbofish orders, stream Tag handling, StackSession, HKDF with a length-last argument, DefaultGenericHash, VecPwHash, increment_bytes, and the new Classic detached-open order.
  • Result: all 2.0 snippets ok.
  • This check changed one row: DryocBox::from_parts(None, ..) cannot infer its ephemeral key type, so those rows use VecBox::from_parts.
• The Rust blocks in UPGRADING.md are not doctests, and CI runs no Markdown checks.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.88%. Comparing base (b284037) to head (3e75e53).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #225      +/-   ##
==========================================
+ Coverage   68.32%   72.88%   +4.56%     
==========================================
  Files         115      115              
  Lines       10032    10032              
==========================================
+ Hits         6854     7312     +458     
+ Misses       3178     2720     -458     

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