expression/SOURCES.md: the prose still said two sources

Adding hpa_consensus left the surrounding text stale -- "two references", "both sources
are measured RNA-seq", a key_type paragraph naming only GTEx and TCGA, and a ~105 MB
download size for a file that is now larger. Found by auditing every count claimed in the
published docs against the files themselves; the counts were right, the prose around them
was not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

expression/SOURCES.md

@@ -1,14 +1,14 @@
 # `expression/` — sources
 
-One file, two references that are **never merged** — different measurements, different units, and a
-gene-keyed table and a peptide-keyed one are not the same object.
+One file, three references that are **never merged** — different measurements, different units, and
+a gene-keyed table and a peptide-keyed one are not the same object.
 
 | | |
 |---|---|
 | file | `reference_expression.tsv.gz` |
 | rows | 6,681,814 |
 | schema | `key`, `key_type`, `source`, `context`, `median_tpm`, `q25_tpm`, `q75_tpm`, `n` |
-| provenance | **experimental** (both sources are measured RNA-seq); the per-context median/IQR/`n` are **computed** summaries of it |
+| provenance | **experimental** (all three sources are measured RNA-seq); the per-context median/IQR/`n` are **computed** summaries of it |
 
 | `source` | `key_type` | rows | contexts | what a row is |
 |---|---|--:|--:|---|
@@ -18,12 +18,13 @@ gene-keyed table and a peptide-keyed one are not the same object.
 
 Total 6,681,814 rows.
 
-`key_type` is the join key and it differs by source on purpose: GTEx answers "is this gene on in
-normal tissue", TCGA answers "is this neoantigen transcribed in this tumour". A caller that treats
-them as one column is asking a question neither answers.
+`key_type` is the join key and it differs by source on purpose: GTEx and HPA answer "is this gene on
+in normal tissue", TCGA answers "is this neoantigen transcribed in this tumour". A caller that treats
+them as one column is asking a question none of them answers. GTEx and HPA are both gene-keyed but
+are still not interchangeable — see the units caveat in the HPA section below.
 
 Read through `mhcmatch.expression` (`lookup`, `tissues`, `tumor_types`, `safety_profile`), which
-keeps the two apart. Fetched by `mhcmatch bootstrap --reference` (~105 MB).
+keeps them apart. Fetched by `mhcmatch bootstrap --reference` (~115 MB).
 
 ---