fix: dedup extracted requirements

[camshaft]

Issue #, if available:

Resolves #74

Problem

The current extraction logic has a bug where it sometimes emits multiple requirement statements for the same text. It mostly comes down to two different scenarios:

Duplicate text

Sometimes a requirement sentence can appear multiple times in a single section.

## Section 1

The implementation MUST do this.

**snip**

The implementation MUST do this.

Compound requirement

Sometimes a requirement can appear in the same sentence as another:

## Section 1

The implementation MUST do this and also MUST do that.

Previously the report command "accidentally" deduplicated these strings when generating the report, since it was using a BTreeSet<Annotation>. With the recent refactors that added line numbers to the Annotation struct, these are no longer considered equivalent.

Solution

This change deduplicates requirements at the extract phase by normalizing the text and checking if it's already been mentioned as a requirement. If it has, then it skips emitting it a second time.

Note that even with this change we still have the gaps described in #159 and #161. Solutions for those will come separately.

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