docs: reference #261 in the changelog and record the behaviour changes

Adds the PR link to the existing entries, which every other entry in the
file carries, plus the two user-visible changes made since: the trivial
exponents of `power` short-circuiting the decomposition, and `exponential`
of a complex hermitian matrix now being exactly hermitian.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: lkdvos

docs/src/changelog.md

@@ -22,18 +22,22 @@ When releasing a new version, move the "Unreleased" changes to a new version sec
 
 ### Added
 
-- New matrix functions `squareroot`, `logarithm` and `power` (with both integer and fractional exponents), supporting the `MatrixFunctionViaLA`, `MatrixFunctionViaEig`, `MatrixFunctionViaEigh` and `DiagonalAlgorithm` algorithms.
-- The scalar type of the output of these matrix functions matches that of the input, and out-of-domain eigenvalues (e.g. on the negative real axis for a real input) throw a `DomainError`; eigenvalues that violate the domain within a tolerance `domain_atol` (defaulting to `default_domain_atol`) are treated as rounding artifacts and clamped to the domain boundary.
-- `MatrixFunctionViaEig` and `MatrixFunctionViaEigh` accept a `domain_atol` keyword argument to control this tolerance.
+- New matrix functions `squareroot`, `logarithm` and `power` (with both integer and fractional exponents), supporting the `MatrixFunctionViaLA`, `MatrixFunctionViaEig`, `MatrixFunctionViaEigh` and `DiagonalAlgorithm` algorithms ([#261](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/pull/261)).
+- The scalar type of the output of these matrix functions matches that of the input, and out-of-domain eigenvalues (e.g. on the negative real axis for a real input) throw a `DomainError`; eigenvalues that violate the domain within a tolerance `domain_atol` (defaulting to `default_domain_atol`) are treated as rounding artifacts and clamped to the domain boundary ([#261](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/pull/261)).
+- `MatrixFunctionViaEig` and `MatrixFunctionViaEigh` accept a `domain_atol` keyword argument to control this tolerance ([#261](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/pull/261)).
 
 ### Changed
 
+- `power(A, p)` resolves the exponents `p = 0` and `p = 1` directly as `I` and `A` without computing a decomposition, so they now apply to any square matrix regardless of the algorithm selected ([#261](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/pull/261)).
+
 ### Deprecated
 
 ### Removed
 
 ### Fixed
 
+- `exponential` of a complex hermitian matrix via `MatrixFunctionViaEigh` is now hermitian to the last bit, rather than only up to rounding errors, matching the guarantee the real case already provided ([#261](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/pull/261)).
+
 ### Performance
 
 ## [0.6.9](https://github.com/QuantumKitHub/MatrixAlgebraKit.jl/compare/v0.6.8...v0.6.9) - 2026-07-10